UC-NRLF 


A  MANUAL  OF  AEROGRAPH! 


m 


FOR  THE 


UNITED  STATES  NAVY 


1918 


FIRST  EDITION 


cud  laforoatiwi  Section 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 
1918 


c 


PREFACE. 

NAVY  DEPARTMENT, 
Washington,  D.  C.,  March  98,  1918. 

This  is  the  first  edition  of  the  Aerographer's  Manual  of  the  United 
States  Navy,  compiled  and  edited  under  the  supervision  of  Lieut. 
Commander  Alexander  McAdie,  U.  S.  N.  R.  F.  The  object  of  this 
book  is  to  aid  in  the  instruction  and  guidance  of  the  personnel  of  the 
Aerography  Division  of  the  United  States  Naval  Reserve  Flying 
Corps.  The  department  invites  criticism  and  suggestions  as  to  the 
form  and  substance  of  the  book.  Such  criticisms  should  be  sent  to 
the  Navy  Department,  Washington,  D.  C.,  Naval  Operations,  Avia- 
tion Division. 

The  following  list  gives  the  names  of  those  who  have  taken  an  active 
part  in  the  preparation  of  the  Manual.  The  initials  indicate  specific 
contributions : 

A.  Me  A Lieut.  Commander  Alexander  McAdie. 

W.  J.  H Prof.  W.  J.  Humphreys. 

C.  J.  P.  C Captain  C.  J.  P.  Cave,  R.  E. 

J.  B.  A Ensign  J.  B.  Anderson. 

R.  F.  B Ensign  Roswell  F.  Barratt. 

J.  \V.  A.  B Ensign  J.  W.  A.  Brown. 

C.  N .  K Ensign  C.  X .  Keyser. 

A.  B.  L Ensign  A.  B.  Leonard. 

A.  S.  M Ensign  A.  S.  Macdonald. 

W.  F.  P Ensign  W.  F.  Prien. 


CONTENTS. 


Page. 

Chapter    1.  Definition  of  units  and  symbols  used 7 

Chapter    2.  Fundamental  equations 17 

Chapter    3.  The  winds 25 

Chapter   4.  Holes  in  the  air 39 

Chapter   5.  Storms  and  storm  tracks 51 

Chapter    6.  Pressure 57 

Chapter    7.  Temperature 69 

Chapter    8.  Water  vapor 77 

Chapter    9.  Radiation 95 

Chapter  10.  Atmospheric  electricity 101 

Chapter  11.  Optical  phenomena 107 

Chapter  12.  Instruments 115 

Chapter  13.  Signals 129 

Chapter  14.  Forecasting 133 

Chapter  15.  Meteorological  conditions  increasing  the  danger  of  flying 141 

Chapter  16.  Tables 153 

Chapter  17.  Bibliography 161 

5 


CHAPTER  I 

DEFINITION  OF  UNITS  AND 
SYMBOLS  USED 


CHAPTER  I. 


UNITS  AND  SYMBOLS. 

In  order  to  meet  the  requirements  of  the  new  meteorology  now 
generally  called  "  aerography, "  the  adoption  of  a  standard  and 
finally  an  international  standard  set  of  recognized  units  is  desirable 
if  not  indeed  essential.  In  selecting  this  new  notation,  the  units 
should  be  such  that  compilation  of  data  will  be  simplified,  that 
fewer  mistakes  will  occur,  and  above  aU  that  definite  and  precise 
conceptions  of  the  phenomena  of  the  atmosphere  shall  be  had, 
particularly  of  the  various  transformations  of  energy  which  are 
manifested  in  general  and  local  disturbances. 

1.  The    centimeter-gram-second     (C.    Q.     S.)     system.— 

(a)  Centimeter  (cm.)  is  the  unit  of  length  and  the  hundredth  part 
of  a  meter.  The  meter  is  generally  defined  as  the  ten-millionth  part 
of  the  meridian  passing  through  Paris.  Lengths  and  distances  will 
be  designated  by  I. 

(6)  Gram  (gm.)  is  the  unit  of  quantity  of  matter  and  is  the  thou- 
sandth part  of  the  quantity  of  matter  in  a  standard  piece  of  platinum- 
iridium  called  the  kilogram-prototype. 

The  kilogram,  or  standard  of  mass,  is  made  as  nearly  as  possible 
equal  to  the  mass  of  a  cubic  decimeter  of  distilled  water  at  maximum 
density  277K  or  1014  on  the  Kelvin-Kilograde  scale.1  Weights  will 
be  designated  by  11',. 

(c)  Second  (sec.)  is  the  unit  of  time  and  is  the  mean  solar  second, 
i.  e.,  there  are  86,400  such  seconds  in  a  mean  solar  day. 

Time  unit  will  be  designated  by  s. 

Derived  from  the  three  foregoing  units  are— 

(d)  Velocity,  the  rate  of  change  in  position  of  a  body  or  the  ratio 

of  length  to  time,  the  conversion  factor  being-- 

s 

Velocity  will  be  designated  by  vel  or  v  and  the  unit  is  1  centimeter 
per  second. 

(e)  Momentum   is   the   quantity   of   motion   and   is   the   product 

Wl 
of  mass  and  velocity,  so  that  the  conversion  factor  is  -  -  =  M . 

1  Kelvin-Kilograde.  a  name  used  it  is  believed  Tor  the  first  time  in  this  work,  to  designate  a  scale  starting 
from  the  absolute  zero  and  having  1,000  for  the  temperature  of  melting  ice  under  standard  conditions 
This  scale  was  suggested  by  Prof.  A.  McAdie  and  described  in  the  Annals,  Harvard  College  Observatory, 
vol.  73,  part  III,  1916.  It  was  then  called,  for  the  lack  of  a  better  name,  the  New  Absolute. 

9 


10  :  MANUAL-  OF  AEROGKAPHY. 

(/f  '-Ac^dCTali6n^*4te  Varfe  of  change  of  the  velocity  of  a  body, 
expressed  as  1  centimeter  per  second  per  second,  and  the  conversion 

factor  will  be  -  =  v8 . 

o 

(g)  The  unit  of  area  will  be  expressed  by  Z2  or  a,  and  the  unit  of 
volume  as  Z3  or  v. 

(h)  The  unit  of  density  or  the  ratio  of  mass  to  volume  is  expressed 

W 

by  — T|>   or  the  Greek  letter  rho(p). 

2.  Force. — The  unit  of  force  is  Called  the  dyne,  i.  e.,  the  force 
which  will  impart  to  the  unit  mass  (a  gram)  an  acceleration  of  1 
centimeter  per  second  per  second. 

Force  is  measured  by  the  rate  of  change  of  momentum,  and  the 

conversion  factor  is  — \  >    designated  by  F. 

9s 

One  pound  in  the  old  system's  unit  of  force  is  13,825  dynes,  when 
gravity  is  not  considered. 

3.  Work.— Work  done  by  a  force  may  result  in  change  of  velocity 
or  change  of  form,  the  former  being  a  change  in  kinetic  energy  and  the 
latter  a  change  in  potential  energy. 

TT7 1 2 
The  unit  of  work  is  the  erg,  the  conversion  factor  — V"   to  be 

«7 

designated  by  TPk.  To  raise  1  kilogram  10  meters  requires  1,000,000 
ergs;  981  X  106  dynes  is  the  value  of  a  poundal  when  the  work  done 
is  against  gravity. 

4.  Power. — The  unit  of  power  is  the  watt,  or  10,000,000  ergs  per 
second. 

In  the  old  system  the  unit  was  a  horsepower,  or  550  foot-pounds 
per  second.  In  the  new  system  the  equivalent  value  will  be  30.48  X 
17, 710  X  13,825  -=-107  =  746  watts  per  horsepower.  A  kilowatt,  or 

1,000  watts,  is  equal  to    L.~   =1.34  horsepower. 

Power  will  be  designated  by  the  symbol  p.  Electrical  engineers 
use  the  letter  "P"  to  represent  power  and  sometimes  the  letters 
KVA,  meaning  the  product  of  the  voltage  and  amperage  divided  by 
1,000.  In  this  manual  the  use  of  P  is  restricted  to  pressure. 

5.  Pressure.  —Atmospheric  pressure  has  been  expressed  in  units 
of  height  of  a  column  of  mercury,  in  units  of  weight,  and,  finally,  in 
units  of  force. 

A  megadyne  atmosphere  or  the  pressure  equivalent  to  the  force 
of  1,000,000  dynes  per  square  centimeter  is  0.987  of  the  standard 
sea-level  pressure  at  latitude  45°  and  freezing  temperature.  It  is 
the  standard  pressure  at  an  elevation  of  106  meters  above  sea  level. 


MANUAL  OF  AEROGRAPHY.  11 

Unit  of  pressure.  The  "bar"  or  1  X  10~6  of  a  standard  atmosphere 
is  the  unit  ofy  pressure  expressed  in  terms  of  force,  or  1  dyne  per 
square  centimeter. 

The  millibar  (mb)  is  the  one-  thousandth  part  of  a  bar;  a  kilobar 
(kb)  is  equal  to  a  thousand  bars,  ar^d  is  the  practical  unit  of  pressure 
for  aerographic  use. 

In  brief: 

Millibar  or  mb  =  I  X  10~9  standard  atmosphere. 
Bar  or  b  =  1  X  10~6  standard  atmosphere. 
Kilobar  or  kb  =  I  X  10~3  standard  atmosphere. 
Megabar  or  mgb  is  the  standard  atmospheric  pressure. 

Unfortunately,  the  term  "millibar"  has  been  inadvertently  used 
by  certain  meteorologists  for  kilobar.  The  change  is  easily  made  by 
substituting  Icb  for  the  symbol  mb  printed  on  maps  previous  to  1918. 

Pressure  will  be  designated  by  P. 

6.  Temperature.  —  For  convenience  in  the  rapid  compilation  of 
data  and  in  notes  relating  to  aerography,  T  will  represent  the  gen- 
eral term  of  temperature  on  any  scale.  Tc  will  indicate  tempera- 
tures Centigrade.  Tf  temperatures  on  the  Fahrenheit  scale,  and  T* 
will  designate  temperatures  on  the  Absolute  scale  devised  by  Lord 
Kelvin,  and  T^  temperatures  on  the  Kelvin-Kilograde  scale. 

THE  MECHANICAL  EQUIVALENT  OF  HEAT. 

By  experiment,  it  has  been  shown  that  the  energy  which  can 
raise  the  temperature  of  a  gram  of  water  1  degree  could  do  the 
mechanical  work  of  lifting,  against  the  force  of  gravity,  1  gram  of 
water  42,683.7  cm.  This  is  called  the  mechanical  equivalent  of 
heat  under  standard  gravity,  i.  e.,  at  45°  latitude  and  at  sea  level, 

and  is  expressed  by  the  symbol  i.    Conversely,  the  "heat  equiva- 

A. 

lent  of  work"  is  expressed  by  A,  its  value  being  0.00002343. 

MEASUREMENTS    OF   HEAT. 

Heat  may  be  measured  in  dynamical  or  thermal  units.     In  dynam- 


ical  units,  the  conversion  factor  is  --^-,  or  E,  energy;  i.e.,  the  tem- 

9s 

perature  of  a  body  may  be  considered  to  be  the  average  kinetic 
energy  of  translation  of  its  molecules  and  is  designated  by  mass  times 
velocity  squared. 

In  thermal  units,  it  is  necessary  to  determine  the  amount  of  heat 
required  to  raise  unit  mass  of  water  1  degree,  the  conversion  factor 
being  WtT. 

The  heat  unit  is  the  gram-calorie  or  therm,  or  the  quantity  of 
heat,  Q,  which  will  raise  the  temperature  of  a  gram  of  pure  water  1 
degree  Centigrade. 


12  MANUAL   OF   AEROGRAPH Y. 

Since  the  specific  heat  of  water  varies  slightly  at  different  tem- 
peratures, the  value  of  the  gram-calorie  is  properly  one  one-hun- 
dredth of  the  total  heat  required  to  raise  the  temperature  of  a  gram 
of  water  from  273K  to  373K  or  1,OOOKK  to  1,366KK. 

7.  International  symbols. *— In  addition  to  the  foregoing  units, 
the  use  of  certain  symbols  was  agreed  upon  by  the  congress  at  Vienna 
in  1873,  and  as  subsequently  modified  are  now  in  use  as  follows: 

<§  Rain.  /  Gale. 

>|<  Snow.  n  Thunderstorm. 

A.  Hail.  <  Distant  lightning. 

A  Sleet.  ^  Duration  of  sunshine. 

=  Fog.  T  Distant  thunder. 

_/m  Dew.  oo  Haze. 

LJ  Hoar  frost.  ©  Solar  halo. 

PXi  Surrounding  country  more  than  half     0  Solar  corona. 

under  snow.  ^  Lunar  halo. 

V  Frostwork  (rough)  forming.  U,  Lunar  corona. 

' — »  Ice  coating  (smooth)  forming.  ^  Rainbow. 

-£»  Drifting  snow.  <^  Aurora. 

< — Floating  ice  crystals.  %  Sea  breeze. 

The  intensity  of  a  phenomenon  is  denoted  by  an  exponent;  0,  indi- 
cating slight;  2,  great;  and  an  absence  of  exponent,  moderate  in- 
tensity. 

The  time  of  occurrence  is  expressed  in  hours  and  tenths:  morning 
and  afternoon  are  indicated  by  A.  and  P.,  respectively;  midnight  and 
noon  by  12  P.  and  12  M.,  respectively,  the  hours  being  counted  from 
0  to  12,  commencing  at  midnight.  The  continuance  of  a  phenomenon 
is  indicated  by  a  dash  ( — ). 

Maximum  and  minimum  values  are  denoted  by  heavy-faced  type 
except  for  relative  humidity,  in  which  case  only  the  minima  are  so 
indicated. 

It  is  desirable  that  a  24- hour  system  be  used  as  lar  as  possible. 
The  beginning  of  the  day  will  correspond  with  0,  Greenwich  mean  civil 
time  and  the  last  hour  marked  23. 

8.  General  geodetic  data.2—!  cm.  equals  0.3937  inch,  or  0.0328 
feet.     (1  mm.  is  roughly  0.04  inch.) 

1  cm.2  equals  0.155  square  inch. 

1  cm.3  equals  0.061  cubic  inch. 

1  cm.  per  second  equals  0.0224  mile  per  hour,  or  0.0328  foot  per 
second;  1  meter  per  second  equals  2.24  miles  per  hour. 

The  equatorial  radius  of  the  earth  is  6,378,388  ±  18  meters,  or  3,963 
miles. 

The  polar  semi-diameter  of  the  earth  is  6,356,909  meters,  or  3,950 
miles. 

The  reciprocal  of  flattening  is  1/297.4. 

1  Taken  from  "  Principles  of  Aerography,"  pp.  31. 

2  Taken  from  "  Principles  of  Aerography,"  pp.  31-32. 


MANUAL   OF   AEROGBAPHY.  13 

The  circumference  of  the  equator  is  40,076,000  meters,  or  24,902 
miles. 

The  perimeter  of  the  meridian  ellipse  is  40,008,600  meters  or  24,860 
miles. 

The  area  of  the  earth's  surface  is  510,044,000  square  kilometers  or 
196,940,000  square  miles. 

The  area  of  the  ocean  is  approximately  hree  quarters  that  of  the 
whole  earth's  surface. 

The  mass  of  the  earth  is  5,984   X   1024  kilograms,  or  6x  1021  tons. 

The  mass  of  the  atmosphere  is  5,263  X  1015  kilograms,  or  5.8X  1015 
tons. 

The  mass  of  the  ocean  is  about  1.3  X  1024  kilograms,  or  1.3  X  1018  tons. 

The  volume  of  the  atmosphere  is  approximately  4,080  X  1015  cubic 
meters;  and  since  a  cubic  meter  of  dry  air  under  standard  conditions 
weighs  1.293  kilograms,  the  approximate  weight  of  the  atmosphere  is 
5,263  X  1015  kilograms.  This  is  1/1,125,000  of  the  mass  of  the  earth. 

If  the  density  of  water  under  standard  conditions  is  taken  as  1, 
then — 

The  mean  density  of  the  earth  is  5.52. 
The  mean  density  of  the  surface  is  2.67. 
The  mean  density  of  the  ocean  is  1.03. 

The  mean  solar  day  is  24  hours,  3  minutes,  56  seconds,  sidereal  time. 
A  sidereal  day  has  86,164  seconds,  or  23  hours,  56  minutes,  4  seco~hds 
mean  solar  time. 

A  sidereal  year  has  365.26  mean  solar  days. 

The  mean  distance  from  earth  to  sun  is  149,500,000  kilometers, 
or  92,900,000  miles. 

Solar  parallax,  8,796  seconds;  lunar  parallax,  3,422.68  seconds. 

Sun's  diameter,  1,392,000  kilometers,  or  865,000  miles. 

The  mean  distance  from  earth  to  moon  is  384,399  kilometers,  or 
238,854  miles,  or  60.3  terrestrial  radii. 

The  velocity  of  light  is  299,870  kilometers,  or  186,300  miles  per 
second. 

The  time  required  for  light  to  traverse  the  mean  radius  of  the 
earth's  orbit  is  498.8  seconds. 

9.  Standard  notation — 

e  Base  of  natural  logarithms,  2.718281828 

TT  3.14159265. 

0  Latitude. 

X  Longitude. 

p  Density. 

CP  Specific  heat  of  air  at  constant  pressure. 

Cv  Specific  heat  of  air  at  constant  volume. 

TT  Ratio  of  specific  heat. 

g  Acceleration  of  gravity  at  sea  level. 


14  MANUAL   OF   AEKOGEAPHY. 

g0  Acceleration  of  gravity  at  a  height  of  106  meters  above  sea 

level.     (The  new  pressure  base.) 

I  Distance  or  height  in  cms. 

Wt  Weight  in  grams. 

s  Time  in  seconds. 

v  Velocity  in  cms.  per  second. 

M  Momentum. 

vt  Acceleration. 

Ms  or  F  Force  in  dynes. 

TFk  Work  in  ergs. 

p  Power  in  watts. 

V  Volume. 

T.  Temperature  (general). 

Tk  Temperature  on  Absolute  scale. 

Tkk  Temperature  on  Kelvin-Kilograde  scale. 

A  Heat  equivalent  of  work  or  0.00002343. 

— r-  Mechanical  equivalent  of  heat  or  42,683.7  gm.  cm. 

Q  Quantity  of  heat,  the  gram-calorie  being  the  unit. 

Ql  Initial  quantity  of  heat. 

Q2  Final  quantity  of  heat. 

Qt-Qz  Free  heat. 

E  Energy. 

U  Inner  energy. 

Ki-Kz  Radiation  energy. 

Si-S2  Entropy  (S  being  the  general  term). 

P  Pressure. 

I  Bar=  1  X  10~6  megadyne  atmosphere. 

ml  Millibar  =  1  X  10~9  bar. 

H  Kilobar  =  1  X  1 0~3  bar. 

mgl  Megabar  =  1  X  106  bars,  standard  atmosphere. 

a  Area. 

N  Avogadro's  constant  =  6.062  X  1023/ 

77  Plancks's  element  of  action. 

M  0.001  mm. 

w  Wave  length. 

G?  Temperature  gradient. 

GP  Pressure  gradient. 

R  Gas  characteristic, 

co  Angular  velocity  of  earth's  rotation. 

r  Radius  of  earth. 

X  YZ  Coordinates  for  axes. 

Conversion  factors  to  be  used  in  connection  with  aerography: 
1  kilowatt-hour  =3,412. 66  B.  t.  u. 
1  h.  p.  =  746  watts. 


MANUAL  OF   AEROGRAPHY.  15 

1  h.  p.-hour  =  2,544.6  B.  t.  u. 

1  B.  t.  u.  =777.5  foot-pounds. 

1  B.  t.  u.  =  0.252  calories. 

1  large  calorie  =  1 ,000  therms. 

1  calorie  =  3.968  B.  t.  u. 

1  calorie  per  kilogram  =  1.8  B.  t.  u.  per  pound. 

1  pound  of  air  at  32°  F.  occupies  about  12.4  cubic  feet. 

1  kg.  of  air  at  273°  A.  occupies  about  0.7741  cu.  m. 

1  pound  of  water  at  212°  F.  occupies  0.0161  cubic  feet. 

1  kg.  of  water  at  373°  A.  occupies  about  8.95  cu.  m. 

1  pound  of  steam  at  212°  F.  occupies  26.14  cubic  feet. 

1  kg.  of  steam  at  373°  A.  occupies  164.1  cu.  m. 

1  pound  of  water  at  212°  F.  contains  181.8  B.  t.  u. 

1  kg.  of  water  at  373°  A.  contains  400.72  B.  t.  u.  or  100.98  calories. 

1  pound  of  steam  at  212°  F.  contains  1,150.4  B.  t.  u. 

1  kg.  of  steam  at  373°  A.  contains  2536.1  B.  t.  u.  or  639.16  calories. 

1  pound  of  ice  requires  143.8  B.  t.  u.  to  change  to  water. 

1  kg.  of  ice  requires  317.02  B.  t.  u.  or  79.89  calories  to  change  to  water. 

1  cubic  foot  of  water  at  212°  F.  weighs  59.84  pounds. 

1  cu.  m.  of  water  at  373°  A.  weighs  0.768  kg. 

1  cubic  foot  of  water  at  62°  F.  weighs  62.2786  pounds. 

1  cu.  m.  of  water  at  290°  A.  weighs  0.800  kg. 

1  cubic  foot  of  steam  at  212°  F.  weighs  0.03826  pound. 

1  cu.  m.  of  steam  at  373°  A.  weighs  0.000522  kg. 

1  cubic  foot  of  dry  air  at  32°  F.  weighs  568  grains. 

1  cu.  m.  of  dry  air  at  373°  A.  weighs  1.293  kg. 

1  cubic  meter  of  dry  air  at  0°  C.  weighs  1,293.05  grams. 

Specific  heat  of  water  1 . 

Specific  heat  of  ice  0.489. 

Specific  heat  of  water  vapor  0.453  at  atmospheric  temperatures. 

Specific  heat  of  air  0.241. 

Values  given  above  are  laboratory  values,  obtained  by  using  dis- 
tilled water.  Ordinary  drinking  water  is  heavier  than  distilled  water, 
because  of  matter  in  solution.  Salt  water  is  also  heavier.  It  may  be 
remarked  that  the  temperature  of  the  freezing  point  in  ordinary  use, 
that  is,  273  K,  may  not  hold  for  the  freezing  of  water  in  plant  life. 
W.  N.  Shaw  instances  one  plant  where  the  freezing  point  is  ap- 
parently 268  K.  In  other  words,  the  change  of  water  from  the  liquid 
to  the  solid  state  under  natural  conditions  is  somewhat  different 
from  the  change  as  studied  in  a  laboratory. 

A.  B.  L. 


CHAPTER  II. 


FUNDAMENTAL  EQUATIONS. 


50821—18 2  17 


CHAPTER  II. 


The  following  formulae  are  considered  as  fundamental  in  connection 
with  the  aerographer's  work. 

Pressure  gradient.  —  As  given  by  the  "dynamic"  definition  the 
vertical  pressure  gradient 

Gp=-pg  (1) 

and  by  the  "  geometric"  definition  as 

GP=-f  (la) 

where  p,  g,  P,  Z  are  in  C.  G.  S.  units 
Gradient  wind. 

GP  =  2o>  v  sin  0  (2) 

Here  v  is  the  velocity  of  the  wind  in  cm.  per  sec.  and  the  other 
symbols  are  of  standard  nomenclature. 

Deflective  force  due  to  earth's  rotation.  —  Deflecting  force: 

F  =  mass  X  acceleration. 

W 

=  —  *  X  2  a?  p  v  sin  0  (3) 

O 

co  =  angular  velocity  of  earth's  rotation  or 

V-  0.00007292 


v  =  velocity  of  air  in  meters  per  second. 

sin  </>=sin  latitude  (Boston  42°  13'  =0.6719) 

(New  York  40°  43'  =0.6524) 

(Washington  38°  53'  =0.6277) 
p  =  density  of  air  =  0.001293  gm.  per  cubic  meter. 

F  =  TQ  megabar  (wind  10  meters  per  second). 

With  moderate  pressure  gradient  at  surface,  gradient  velocity  is 
reached  about  350  meters  and  gradient  direction  about  700  meters. 
In  light  anti-cyclonic  winds,  gradient  velocity  is  not  reached  below 
1,000  meters. 

Relation  of  pressure  (P)  and  altitude  (I).—  Let  P'2  and  P\  be 

ures  at  heights  under  consideration. 

19 


20  MANUAL  OF   AEROGRAPH Y. 

P2,  P!  and  Tto,  Tkl  the  pressure  and  temperatures  at  the  stations 
Sta.2  and  Sta.,  at  the  ground,  then— 

i  -  Tk2\  (. 

273     ^ 

where  the  pressures  are  measured  in  kilobars. 

Relation  of  velocity  (v)  to  altitude  (1). — Humphreys  has  shown 
that  near  the  earth  (0.2  to  8  m.)  wind  conditions  are  as  follows: 

(1)  Actual  velocity  is  very  irregular, 

(2)  Average  velocity  increases  rapidly  with  elevation, 

(3)  Rate  of  velocity  increase,  (a)  decreases  with  average  velocity; 
(6)  decreases  with  elevation.     And  that  above  this  layer  the  increase 
in  velocity  is  regular  enough  to  allow  its  computation  by  means  of 
such  formulas  as  Stevenson's,  which  is  fairly  accurate  up  to  16  m. 

(5) 


+  22 

Where  vl  is  the  computed  velocity  at  height  Z  from  the  known  veloc- 
ity (v)  at  height  Z. 

For  elevations  varying  from  100  m.  to  600  m.  the  following  formulas 
give  results  which  are  quite  accurate. 

Douglas's  formula: 

r)*  (5a) 

Where  v^  =  velocity  sought  at  height  ?t  and  v  =  velocity  by  anemometer 
at  height  Z. 

Shaw's  formula: 

Z  +  constant 


* 

1      constant 

the  constant  depending  upon  surroundings  and  is  found  for  each 
station. 

Other  interesting  generalizations  concerning  the  variations  of  wind 
with  altitude  have  been  obtained  by  Cesare  Fabris,  who  carried  on 
his  observations  with  pilot  balloons  near  Rome.  He  found  that,  in 
general— 

(1)  The  air  currents  for  the  first  600  m.  to  700  m.  are  greatly 
influenced  by  surface  conditions,  but  that  there  is  a  rapid  increase 
in  velocity  as  the  upper  limit  is  approached, 

(2)  For  a  short  distance  of  100  m.  above  the  first  600  to  700  meters 
there  is  a  decrease  of  velocity  with  altitude, 

(3)  From  500  m.  or  600  m.  to  1,500  m.  altitude  the  winds  are 
irregular,  but  tending  to  increase  in  velocity, 

(4)  Above  1,500  m.  to  the  stratosphere  there  is  a  constant  increase 
in  velocity  with  altitude. 


MANUAL  OF   AEROGRAPHY.  21 

In  his  studies  of  the  upper  atmosphere  Engell  suggested  the 
theory  that  the  volume  of  air  passing  a  given  vertical  plane  per  unit 
of  time  is  the  same  for  all  altitudes.  From  this  it  follows  that 


pv  = 

where  v  and  v,  =  the  wind  velocities  and   p  and  pl  represent  the 
densities  of  the  air  at  the  different  heights. 

In  summing  up  the  general  relations  of  wind  velocity  to  height, 
Humphreys  shows  that  from  5  km.  to  10  km.  the  temperature  is 
nearly  constant,  so 


as 

p  =  ^  (roughly) 
then 


7/ 

-,  =  -j  (roughly) 


from  the  above — 


so 


. 

P'~V 


therefore  — 

T     tir 

T-V  w 

approximately,  or  the  velocity  of  the  wind  through  the  levels  in 
question  is  roughly  proportional  to  the  altitude. 

Above  the  isothermal  layer  the  temperature  gradient  decreases 
so  the  value  of  pv  falls  off  faster  than  the  density,  with  increased 
height.  This  agrees  with  the  mathematical  deduction  that  the 
maximum  velocity  occurs  slightly  below  this  level,  namely,  8  or  9 
kilometers  above  the  surface  of  the  earth. 

Relation  of  temperature  (T)  and  altitude  (Z).  —  From  extensive 
experiments  carried  out  by  different  types  of  balloons  and  airplanes 
the  following  conclusion  may  be  expressed,  that  the  aviator  may 
expect  a  decrease  of  six-tenths  of  a  degree  Centigrade  for  every 
increase  in  altitude  of  roughly  100  meters;  i.  e.,  the  normal  decrease; 
while  the  adiabatic  rate  of  decrease  of  one  degree  Centigrade  corre- 
sponds roughly  to  every  increase  of  100  meters  elevation. 

Relation  between  pressure,  temperature,  and  altitude.— 
In  a  chart  recently  devised  by  Lieut.  Commander  Alexander  McAdie, 
the  airman  has  a  quick  way  of  obtaining  the  true  altitude  corrected 
for  temperature,  the  correction  for  latitude  and  gravity  change  for 
altitude  being  neglected. 


22  MAXUAL   OF   AEROGRAPH  Y. 

Reduction  of  barometric  readings  to  sea  level.  —  Laplace  gives 
the  following  formula;  only  the  symbols  used  here  are  of  standard 
nomenclature  and  values: 


Where— 

1  =  altitude  of  the  station  above  the  sea  level. 
</>  =  latitude. 

r  =  mean  terrestrial  radius,  6,  378,388  meters. 
a  =  the  coefficient  of  expansion  of  air. 

Tk  =  the  equivalent  mean  temperature  on  the  absolute  scale  of  the 
air  between  the  station  and  a  place  supposed  to  be  situated  in  the 
same  vertical  at  sea  level. 

P  and  P0  =  the  atmospheric  pressures  at  the  two  points. 
0.00259  =  the  constant  for  variation  of  gravity  with  latitude. 
R=  barometric  constant,  and  equals 

px     normal  barometric  height  =  76  cm 
PX  loglQe  =  0.43429 

Where— 

Pi  =  density  of  mercury  at  273  K. 
p  =  density  of  air  at  273  K  and  normal  pressure. 
^°#io0  =  modulus  of  common  logarithms. 
Whence  R  =  18,400  meters  =  60,370  feet. 

So  that  by  substituting  the  values  of  these  known  constants  and 
observed  conditions,  the  reduction  of  the  known  pressure  to  that  at 
sea  level  can  be  made.1 

The  equation  of  motion  for  the  atmosphere.1  —  If  the  influence 
of  vertical  currents  and  of  viscosity  may  be  ignored  we  can  write 
down  the  equations 

1  dP  ,, 


Where  P  is  the  pressure,  dl  is  an  element  of  the  path  of  the  air, 
dl'i,  is  at  right  angles  thereto  and  toward  the  left,  p  is  the  density 
of  the  air,  and  r\!  is  the  radius  of  curvature  to  the  left  of  the  path  of 
the  air. 

The  characteristic  equation  for  air.1 

P  =  RpTk  (10) 

for  pressure  in  kilobars,  temperature  in  degrees  Absolute  (Kelvin 
scale)  and  density  in  grams  per  cubic  centimeter: 

For  dry  air  R  =  2,870. 

1  Computer's  Handbook,  1916. 


MANUAL  OF  AEEOGBAPHY.  23 

For  air  saturated  at  273  K  with  a  vapor  pressure  of  6.10  Kb. 
R  =  2,876. 

For  air  saturated  at  283  K  with  a  vapor  pressure  of  12.24  Kb. 
R  =  2,883. 

Weight  of  vapor  in  the  air.1 

Wt  =  P«p,^r  (11) 

Where  Wt  is  the  weight  of  vapor  in  a  cubic  meter  *of  air. 

P'T'k  are  the  standard  pressure  and  temperature  of  the  atmosphere. 

p  =  weight  of  one  cubic  meter  of  dry  air  under  standard  conditions 
of  pressure  and  temperature  and  is  equal  to  1,292.8  grams  per  cubic 
meter. 

6  =  ratio  of  the  density  of  water  vapor  to  air  at  the  same  tempera- 
ture and  may  be  taken  equal  to  0.622. 

P  =  pressure  of  water  vapor  in  the  air. 

Tk  =  temperature  of  the  Kelvin  scale. 

The  computation  of  humidity.1 

T-Td  =  C(T-TO  (12) 

Where — 

T  is  the  temperature  of  the  dry  bulb. 

T'  is  the  temperature  of  the  wet  bulb. 

Td  is  the  temperature  of  the  dew  point. 

C  is  a  factor  which  depends  on  the  temperature  of  the  dry  bulb. 

Glaisher's  hygrometric  tables,  which  are  generally  used  in  the  British 
Isles,  are  especially  prepared  for  this  subject. 

Density  of  air  for  various  pressures.2 

(P  — 3  P'\ 
^rr  /  (13) 

Where — 

p  is  the  density  in  grams  per  cubic  meter. 

P  is  the  pressure  of  the  air  in  kilobars. 

P'  is  the  pressure  of  the  water  vapor  present  in  the  air  in  kilobars. 

Tc  is  the  absolute  temperature  on  the  Centigrade  scale. 

Variation  of  wind  velocity  with  height.2 — It  has  been  found 
by  experiment  that  the  velocity  of  the  wind  increases  with  height 
and  tends  to  gradually  become  parallel  to  the  isobars  in  the  lower 
levels. 

i  Computer's  Handbook. 

2 Air  Navigation  for  Flight  Officers.    (Lieut.  Commander  A.  E.  Dixie,  R.  N.) 


24 


MANUAL  OF  AEROGRAPHY. 


The  veering  of  the  wind  with  height  may  be  roughly  estimated  in 
degrees  from  the  following  formula,  where  "V"  is  the  veering  and 
MH"  the  height,  in  meters: 

H 


SOX 


v= 


1000 


H 


(H) 


1000 


+  2 


Height. 

Veering. 

0 

0° 

1.000 

10° 

2,000 

15° 

3,000 

18° 

4,000 

20° 

5,000 

21° 

6,000 

22  J° 

7,000 
8,000 

i* 

9,000 

24  J° 

10,000 

25° 

11,000 

25  \° 

12.000 

25T 

Fluctuation  and  gustiness.1 — The  velocity  of  the  wind  is  seldom 
uniform,  but  varies  in  gusts  and  lulls. 

The  difference  between  the  average  maximum  velocity  of  the  gusts 
and  the  average  minimum  velocity  of  the  lulls  is  known  as  the 
"fluctuations  of  the  wind." 

The  gustiness  of  the  wind  is  found  as  follows: 

Fluctuations. 


Gustiness  = 


Average  velocity. 
Let  vmax  be  the  maximum  and  vm  the  minimum  velocity. 


Then  gustiness  = 


It  has  been  found  that  the  gustiness  of  the  wind  at  any  particular 
place  for  a  given  direction  is  practically  constant. 

A.  B.  L. 


Air  Navigation  for  Flight  Officers.    (Lieut.  Commander  A.  E.  Dixie,  R.  N.) 


CHAPTER  ffl. 


THE  WINDS. 


25 


CHAPTER  III. 
THE  WINDS.1 

Edmund  Halley,  the  astronomer,  in  1698  received  from  King 
William  III  command  of  a  sailing  ship,  with  directions  to  study 
variations  of  the  compass.  He  made  two  memorable  voyages  and 
practically  covered  the  Atlantic  from  50°  N.  to  50°  S.  Besides  the 
magnetic  work  much  meteorological  work  was  done,  and  our  first 
knowledge  of.  the  general  wind  system  of  the  Atlantic  comes  as  a 
result  of  these  voyages. 

In  1856  another  explorer,  Capt.  Charles  Wilkes,  of  the  United 
States  Navy,  read  a  paper  before  the  American  Association  for  the 
Advancement  of  Science,  which  was  later  published.  It  is  in  this 
work  that  we  find  included  one  of  the  earliest  maps  of  the  winds 
of  the  world.  In  1875,  through  the  joint  agency  of  the  Smith- 
sonian Institution  and  Prof.  J.  H.  Coffin,  of  Lafayette  College,  there 
was  published  a  large  volume  on  the  Winds  of  the  Globe.  Several 
world  maps  of  wind  movement  are  given;  and  not  only  the  annual 
direction  but  the  directions  for  summer  and  winter  months  are 
charted.  Koppen,  of  Hamburg,  has  given  us  the  latest  of  these 
charts. 

It  is  apparent  that  there  are  several  mighty  streams  of  air  flowing 
around  the  world  in  certain  latitudes.  Some  blow  steadily  and  are 
more  or  less  permanent,  like  the  trades,  the  anti-trades,  and  the 
prevailing  westerlies.  Some  are  seasonal  in  character,  like  the 
monsoons.  There  are  also  well-marked  minor  circulations,  known 
as  sea  breezes  and  valley  winds;  and,  finally,  there  are  the  individual, 
localized  winds  accompanying  the  various  storm  types. 

The  permanent,  or  planetary,  winds  are  controlled  by  the  plane- 
tary pressure  distribution;  that  is,  they  depend  upon  the  general 
difference  of  temperature  between  equatorial  and  polar  regions,  and 
more  especially  upon  the  position  and  strength  of  the  great  planetary 
pressure  belts.  The  seasonal  winds  can  be  correlated  with  move- 
ments of  the  hyperbars  or  infrabars  (the  so-called  centers  of  action). 
The  local  winds  can  be  traced  to  temporary  disturbances  of  pressure. 

The  winds  have  been  classified  by  Dove,  Davis,  and  others  as 
planetary,  terrestial,  continental,  land  and  sea  breeze,  mountain 
and  valley  breeze,  cyclonic,  and  certain  accidental  winds  due  to 
volcanic  eruptions. 

Trade  winds. — These  are  the  great  northeast  and  southeast 
wind  systems.  The  name  is  derived  from  the  old  English,  to  blow 

i  Principles  of  Aerography,  Chap.  X.    (A.  McAdie.) 

27 


28  MANUAL   OF  AEROGRAPHY. 

trade,  meaning  in  one  direction.  On  the  pilot  charts  issued  by  the 
Hydrographic  Office,  founded  upon  the  researches  made  and  the 
data  collected  by  Lieut.  M.  F.  Maury,  U.  S.  N.,  there  is  published 
the  average  condition  of  wind  and  weather  for  the  given  period. 
Thus  if  we  look  on  the  chart  of  the  North  Pacific  Ocean  for  May 
we  find  that  the  northeast  trades,  force  4  to  5  (5  to  8  meters  per 
second),  extend  to  within  about  5°  of  the  American  coast  between 
the  twenty-fifth  and  fifteenth  parallels.  They  average  24  days  in 
May  over  the  Hawaiian  Islands.  On  the  other  hand,  the  south- 
east trades,  force  3  to  4  (3  to  5  meters  per  second),  extend  1°  to  5° 
north  of  the  Equator,  and  are  farthest  north  between  longitudes 
150°  W.  and  110°  W. 

Over  the  Atlantic  during  May  we  find  that  the  northeast  trades 
extend  northward  slightly  beyond  the  Canary  Islands,  but  west  of 
the  thirtieth  meridian  the  northern  limit  of  these  winds  is  nearly 
along  the  twenty-fifth  parallel.  The  southern  limit  is  close  to  the 
Equator  on  the  American  side,  but  rises  to  latitude  12°  N.  at  longitude 
20°  W.  The  force  of  the  northeast  trades  is  4  to  5,  increasing  toward 
the  south.  Their  direction  is  northerly  off  the  African  coast,  but 
is  northeast  between  the  twentieth  and  thirtieth  meridians.  Farther 
westward  the  direction  is  more  easterly,  and  north  of  the  Lesser 
Antilles  it  is  southeasterly,  showing  the  anti-cyclonic  circulation 
around  the  Azores  hyperbar.  The  winds  are  generally  east  to 
northeast  in  the  Caribbean  Sea  and  east  to  southeast  in  the  Gulf 
of  Mexico.  The  southeast  trades,  force  3  to  4,  extend  from  1°  to 
30°  above  the  Equator  between  the  eighth  and  forty-second  meridians. 

Over  the  Indian  Ocean  during  May  the  southeast  trades  prevail 
over  the  area  between  the  Equator  and  latitude  30°  S.  Over  the 
extreme  northern  and  southern  portions  of  this  area  the  trades  are 
broken  by  variable  winds,  and  calms  are  frequent  between  the 
Equator  and  10°  S.  The  trades  are  steadiest  between  latitudes  10°  S. 
and  25°  S.  Along  the  African  coast,  near  the  Equator,  they  follow 
the  contour  of  the  land  and  merge  into  the  southwest  monsoon. 

If  we  follow  the  trades  during  winter  months  we  shall  find  that 
over  the  North  Pacific  the  northeast  trades  reach  their  most  northern 
limit  in  the  eastern  part  of  the  ocean  at  the  twenty-ninth  parallel, 
slightly  southeast  of  the  central  area  of  the  California  high,  and  are 
strongest  and  steadiest  south  of  this  region.  Between  longitudes 
145°  W.  and  155°  E.  their  northern  limit  is  close  to  the  25th  parallel. 
They  extend  eastward  to  within  5°  to  8°  of  the  American  coast  and 
westward  to  Asiatic  waters,  where  they  merge  into  the  northeast 
monsoon.  They  extend  as  far  south  as  the  Equator  west  of  longitude 
170°  E.  East  of  this  longitude  their  southern  limit  gradually  rises 
to  the  tenth  parallel  at  longitude  125°  W.  The  southeast  trades 
extend  north  of  the  Equator  between  longitudes  85°  W.  and  180°  W. 


MANUAL   OF  AEROGRAPHY.  29 

They  reach  their  most  northern  limit,  the  sixth  parallel,  between 
longitudes  115°  W.  and  125°  W.  In  the  North  Atlantic  the  north- 
east trades  prevail  between  the  fifth  and  twenty-fifth  parallels. 
Near  Bra/il  they  extend  as  far  south  as  the  Equator,  and  near  the 
African  coast  as  far  north  as  latitude  32°  N.  These  winds  are  the 
typical  northeast  trades  over  the  eastern  part  of  the  ocean  and  in 
the  Caribbean  Sea.  In  the  central  part  of  the  ocean  they  become 
east -northeasterly.  Southeast  trade  winds  extend  north  of  the 
Equator  over  the  central  part  of  the  ocean  to  the  fourth  parallel. 

In  the  South  Atlantic  the  southeast  trades  prevail  from  the  area 
of  high  pressure  to  latitude  5°  S.  on  the  eastern  part  of  the  ocean, 
and  from  latitude  15°  S.  to  the  Equator  on  the  western.  Over  the 
greater  part  of  this  area  they  are  well  developed,  blowing  from  the 
southeast  from  50  to  60  per  cent  of  the  time,  with  a  small  percentage 
of  calms  and  no  gales,  the  average  force  being  about  4.  South  of 
the  area  of  high  pressure  "the  brave  west  winds"  prevail.  They 
have  increased  slightly  in  intensity  since  the  spring.  The  winds 
around  the  high  show  their  anticyclonic  movements  very  plainly, 
while  those  within  the  area  are  variable  in  direction  and  force. 
Over  the  Indian  Ocean  in  winter  the  southeast  trades  occur  between 
10°  S.  and  30°  S.  east  of  the  fiftieth  meridian.  Their  average  force 
is  3  to  4.  West  of  Madagascar  the  winds  are  mostly  northeasterly 
and  southeasterly,  while  south  of  Madagascar  they  are  easterly. 

In  discussing  the  planetary  circulation  it  is  assumed  that  in  the 
upper  levels  there  must  be  an  overflow  of  air  from  the  Equator  to  the 
poles.  Recent  soundings,  however,  do  not  confirm  this  view.  And 
this  variability  in  flow  is  shown  in  marked  degree  above  the  trades. 
The  trades  themselves  are  comparatively  shallow  streams,  not  extend- 
ing above  the  5-kilometer  level.  Above  these  the  air  movement  is 
from  west  to  east;  and  these  winds  are  called  the  anti-trades,  some- 
what unfortunately  since  this  term  is  applied  to  the  winds  farther 
north  or  south,  better  described  as  the  prevailing  westerlies.  We 
shall  use  the  term  ''countertrades"  for  the  winds  above  the  trades. 
The  countertrades,  then,  are  above  the  trades  and  extend  approxi- 
mately from  4  to  16  kilometers,  or  more  than  twice  the  depth  of  the 
surface  trades.  Still  higher  and  above  the  countertrades  flows  the 
so-called  upper  easterly  current,  extending  up  to  a  height  of  20 
kilometers,  and  above  this  again,  a  westerly  flow  in  the  same  direc- 
tion as  the  countertrades,  and  approximately  5  kilometers  in  depth. 
Finally  al  a  height  of  30  kilometers  there  would  seem  to  be  another 
easterly  current.  Thus  over  the  Tropics  we  find  at  least  five  wind 
systems.  As  we  move  to  higher  latitudes,  the  winds,  possibly  under 
the  influence  of  the  deflective  tendency,  due  to  the  earth's  rotation, 
change  their  direction  through  the  south  and  become  eventually 
we>t  winds.  These  air  streams  are  drier  and  heavier  than  the  trades 


30  MANUAL   OF  AEBOGKAPHY. 

and  descend  to  the  surface  in  latitude  30°  from  the  thermal  Equator 
as  warm,  dry,  southwest  winds.  In  the  United  States  the  anti- 
trades are  more  commonly  called  the  prevailing  westerlies,  winds 
which  lack  the  steadiness  of  the  trades,  but  which  nevertheless  are 
the  controlling  factors  in  determining  the  weather  of  the  temperate 
zones. 

In  connection  with  the  movement  of  the  upper  air  it  is  of  interest 
to  note  that  the  dust  from  the  Krakatau  eruption  in  1883,  a  few  de- 
grees south  of  the  equator,  was  carried  from  east  to  west  around  the 
world  in  about  15  days.  The  red  sunsets  and  sunrises  due  to  the 
fine  dust  and  vapor  particles  appear  progressively  later  from  west  to 
east  and  indicated  an  average  movement  of  113  kilometers  per  hour, 

31  meters  per  second. 

Monsoons. — The  word  "monsoon"  is  said  to  be  of  Arabic  origin, 
meaning  "  season,"  and  rightly  applies  to  the  winds  of  the  Indian 
Ocean,  for  the  general  character  of  the  season  and  the  crop  yield  are 
closely  connected  with  the  duration  and  intensity  of  these  winds. 
During  the  summer  months  the  southwest  monsoon  force  3  to  5  (3  to 
8  meters  per  second)  dominates  the  ocean  north  of  the  equator.  It 
overspreads  the  Arabian  Sea  early  in  June,  and  by  the  third  week  is 
in  full  force  over  the  Bay  of  Bengal.  Severe  thunderstorms,  thick, 
cloudy  weather,  and  gales  with  occasional  dangerous  cyclones  occur 
during  the  period  immediately  preceding  the  full  force  of  the  mon- 
soon. In  winter  we  have  to  deal  with  the  northeast  monsoon,  force 
3  to  4  (3  to  5  meters  per  second),  which  prevails  over  Indian  waters 
and  extends  down  the  African  coast  to  latitude  10°  S.  North- 
westerly winds  prevail  in  the  Persian  Gulf  and  the  Gulf  of  Oman,  and 
easterly  winds  in  the  Gulf  of  Aden.  In  the  southern  part  of  the  Red 
Sea  the  winds  are  southeasterly,  and  in  the  northern  part  they  are 
northwesterly.  On  the  Asiatic  coast  the  winds  east  of  Chosen 
(Korea)  are  northeasterly;  west  of  it  they  are  northwesterly.  Along 
the  China  coast  immediately  north  of  Shanghai  to  the  fifth  parallel 
they  are  northeasterly  and  are  known  as  the  northeast  (winter)  mon- 
soon. The  monsoon  is  in  full  force  during  January  and  blows  with 
greatest  strength  and  constancy  between  Macao  and  Chusan.  It 
shows  a  marked  tendency  to  follow  the  coast,  and  as  it  weakens  at 
night  and  the  wind  becomes  somewhat  offshore,  northbound  sailing 
vessels  may  then  make  fair  headway.  The  thick,  rainy  weather  of 
the  monsoon  period  renders  navigation  difficult  off  the  coast  of 
Taiwan  (Formosa).  A  rising  pressure  foreruns  an  increase  in  the 
strength  of  the  monsoon,  and  a  falling. pressure  a  decrease. 

Local  winds. — In  nearly  every  land  there  are  local  names  for 
special  winds,  based  as  a  rule  upon  the  w^arm  or  cold  and  wet  or  dry 
character  of  the  wind.  In  mountainous  countries,  especially  if  the 
range  is  but  a  short  distance  from  some  large  water  surface,  the  air 


MANUAL  OF  AEROGRAPHY.  31 

at  times  seems  to  rush  through  the  valleys  and  canyons.  This  can 
nearly  always  be  traced  to  the  passage  of  some  general  disturbance. 
There  is  another  class  of  day  and  night  winds  which  are  due  prima- 
rily to  differences  of  temperature  in  the  valley  and  at  the  level  of  the 
mountain  tops;  also  sometimes  to  differences  in  the  heating  of  the 
oast  and  the  west  sides  of  the  range.  These  are  the  well-known 
mountain  and  valley  winds,  reversing  their  direction  with  the  change 
from  night  to  day.  In  all  these  wind  systems  the  contour  of  the 
land  plays  an  important  part.  Study  of  the  topography  shows  that 
the  drafts  are  localized  and  intensified  by  the  lay  of  the  land.  Most 
of  these  winds  are  in  the  nature  of  forced  drafts,  in  the  sense  that  air 
masses,  generally  with  moderate  momentum,  forced  through  re- 
stricted channels,  such  as  mountain  passes  and  valleys,  are  drafts. 
Those  are  chiefly  horizontal  currents,  while  the  regular  mountain 
and  valley  winds  are  more  often  due  to  vertical  currents. 

It  is  not  surprising,  therefore,  to  find  that  the  direction  of  the  flow 
may  be  determined  to  some  degree  by  the  trend  of  the  narrow  air 
passage  or  valley.  Thus,  although  the  foehn  wind  in  the  Alps  is 
primarily  a  south  or  southwest  wind,  it  ma}^  appear  in  certain  dis- 
tricts as  a  southeast  wind,  having  had  its  direction  of  flow  deflected 
by  the  trend  of  the  valley.  Furthermore,  displacement  at  one  place 
means  motion  at  some  other  point  of  the  circuit,  and  we  may  have 
an  endless  chain,  as  it  were,  in  which  the  natural  flow  is  masked. 
And  just  as  in  the  case  of  the  flow  of  water  in  rivers  there  may  be 
established  return  currents  at  the  sides  of  the  main  stream,  or  eddy 
currents  at  points  where  obstruction  to  the  general  flow  is  met,  in  the 
central  part  of  a  valley  the  flow  of  air  or  wind  may  be  in  one  direction, 
while  on  the  sides  the  flow  may  be  in  an  opposite  direction.  An 
excellent  way  of  studying  the  flow  of  air  in  mountainous  countries 
is  from  a  station  on  the  summit.  Close  observation  of  the  clouds 
above  and  the  fogs  below,  as  they  form  and  dissipate,  will  show  the 
existence  of  many  unsuspected  air  streams. 

Of  all  the  special  winds,  the  foehn  is  perhaps  the  best  known,  as  it 
has  been  most  studied  and  written  about.  The  word  is  of  German 
origin  and  possibly  is  connected  with  the  Latin  "favonius,"  a  west 
wind  of  the  spring;  but  if  such  were  the  original  meaning  it  is  not  in 
accord  with  the  conditions  now  existing,  for  the  foehn  is  essentially 
a  dry  south  wind.  It  blows  on  the  northern  slopes  of  the  Alps  and  is 
most  noticeable  in  those  valleys  which  have  a  north-and-south  trend ; 
indeed,  it  is  hardly  noticeable  in  some  valleys  which  extend  east  and 
west.  For  many  years  it  was  explained  as  originating  in  the  deserts 
of  Africa;  but  it  is  now  known  to  be  the  southerly  component  of  an 
indraft  due  to  the  passage  of  a  cyclonic  area  over  Western  Europe. 
The  word  foehn  is  also  used  in  a  broader  sense  to  designate  any  wind 
system  where  the  air,  moving  into  a  cyclone,  is  forced  over  some 


32  MANUAL   OF  AEKOGBAPHY. 

range  and  thus  cooled  and  dried;  and  then,  descending  on  the  farther 
slopes,  is  dynamically  heated.  Under  such  conditions  evaporation 
is  rapid  and  snow  on  the  ground  disappears  quickly.  In  the  Northern 
Hemisphere  such  winds  in  temperate  latitudes  are  generally  from 
the  south,  and  in  the  Southern  Hemisphere  from  the  north.  Thus 
we  find  foehn  winds  in  Greenland,  Iceland,  Eastern  Europe,  as  in 
Hungary  (rotenturmwind) ,  South  America,  Japan,  Peru,  and  in  fact 
in  all  parts  of  the  world  where  mountains  act  as  partial  barriers  to  the 
flow  of  air  and  there  is  compression  after  expansion.  Such  a  wind  is 
the  chinook  (name  of  an  Indian  tribe  dwelling  on  Puget  Sound) ,  a 
dry  and  relatively  warm  wind  of  Wyoming,  Montana,  Idaho,  eastern 
Oregon,  and  parts  of  Colorado.  A  good  illustration  of  the  pressure 
distribution  and  resulting  wind  direction  and  temperature  can  be 
found  on  the  weather  map  for  January  23,  1907.  The  temperature 
rose  quickly  from  about  255°  A.  to  275°  A.  and,  as  previously  stated, 
the  snow  evaporated  rapidly.  The  duration  of  the  wind  depends 
upon  the  movement  of  the  low-pressure  area  to  the  north.  Some- 
times the  high  temperature  will  last  24  hours.  Other  warm,  dry 
winds  are  the  so-called  "hot  winds"  of  the  Plains  States,  the  summer 
winds  of  Texas,  the  " northers"  of  the  Sacramento  and  San  Joaquin 
valleys,  and  the  " Santa  Anas"  of  southern  California.  Some  of 
these  winds  in  the  summer  months  pass  over  heated  areas  and  are 
warmed  to  some  degree  by  radiation  from  the  earth.  The  sirocco  of 
southern  Italy  and  Greece  is  a  warm  south  wind,  generally  dust-laden 
and  therefore  trying  to  man  and  beast.  Theleveche  of  Spain  and  the 
leste  of  the  Maderia  Islands  are  sirocco  winds;  the  solano  is  an  east 
wind  on  the  east  coast  of  Spain;  the  harmattan  is  a  hot,  dusty  east 
wind  of  the  winter  months  in  the  Gulf  of  Guinea;  the  simoon  (from 
the  Arabic  word  to  poison,  although  there  are  no  poisonous  gases 
associated  with  it)  is  a  hot,  sand-laden  wind  felt  in  Palestine,  Syria, 
and  Arabia;  the  Khamsin  of  Egypt  is  a  hot  southeast  wind  which 
blows  for  about  50  days  after  the  middle  of  March;  the  brickfieiders 
are  hot  north  winds  of  Southern  Australia.  There  are  many  others 
having  local  names. 

Cold  waves  and  boreal  winds. — The  word  "boreal,"  from 
"Boreas,"  the  north  wind  of  the  Greeks,  is  used  to  designate  a  class  of 
cold  winds  generally  of  cyclonic  origin.  There  would  seem  to  be 
some  connection  between  the  intensity  of  the  depression  and  the 
temperature  of  the  northwest  quadrant.  Being  preceded  by  com- 
paratively warm  southerly  winds,  the  contrast  is  marked  and  all  the 
more  noticeable.  The  air  is  not  necessarily  brought  from  high  levels, 
and  the  compression  is  not  sufficiently  great  to  warm  the  air  enough 
to  affect  materially  its  initial  low  temperature.  The  so-called  cold 
waves  of  the  United  States  are  essentially  boreal  winds.  Such,  too, 
are  the  "buran"  or  "purga"  of  Russia,  the  "pamperos"  of  Argen- 


MANUAL   OF  AEROGRAPH Y.  33 

tinn.  the  southerly  "burster"  of  New  Zealand,  the  "bora"  of  the 
Adriatic,  and  the  "mistral"  of  the  valley  of  the  Khone.  The  word 
"mistral"  is  derived  from  the  Latin  "magister,"  and  the  wind  is 
therefore  appropriately  described  as  a  master  wind,  or  the  wind 
which  dominates.  It  has  been  known  for  some  time  that  the  winds 
of  the  Antarctic  region  were  of  higher  velocity  and  lower  temperature 
than  elsewhere,  and  the  records  of  the  Australian  Antarctic  expedi- 
tion of  1911-1914  confirms  this.  Thus  gusts  of  wind  having  a 
velocity  of  nearly  90  meters  per  second  (200  miles  per  hour)  were 
recorded  on  the  Robinson  anemometers  used.  The  record  is  subject 
to  correction,  and  these  figures  may  be  reduced  20  per  cent. 
Velocities  of  80  meters  per  second  and  even  higher  were  not  infrequent, 
nor  were  winds  of  45  meters  per  second  (100  miles  per  hour)  with  a 
temperature  of  240°  A.  (-28°  F.)  rare. 

Charts  for  aeronauts  and  aviators. — The  term  "aeronaut"  is 
used  to  designate  the  pilot  of  a  balloon,  while  "aviator"  is  restricted 
to  the  pilot  of  a  heavier-than-air  flying  machine.  One  of  the  first 
attempts  to  bring  the  results  of  the  exploration  of  the  air  by  kites, 
balloons,  and  other  means  into  convenient  form  for  the  use  of 
aviators  and  aeronauts  is  the  volume  by  Rotch  and  Palmer,1  issued  in 
1911.  Charts  of  the  relative  heights,  corresponding  densities,  and 
temperatures  are  given.  The  frequency  of  winds  from  various 
directions  and  their  respective  velocities  at  Blue  Hill  are  shown. 
Thus  the  shallowness  of  easterly  winds  is  made  evident  by  comparison 
at  different  levels.  The  summer  sea  breeze  has  a  depth  of  about 
1,000  meters,  while  the  easterly  winds  of  cyclonic  origin  may  have  a 
depth  of  2,000  meters.  The  winds  of  winter  are  of  greater  velocity 
than  the  winds  of  summer.  In  brief,  west  winds  are  most  frequent. 
Near  the  ground  they  blow  about  25  per  cent  of  the  time  from  south- 
southwest  to  west-southwest  in  summer,  and  about  33  per  cent  of  the 
time  from  west  to  northwest  in  winter,  with  a  velocity  varying  from  8 
to  11  meters  per  second.  At  a  height  of  3,000  meters  the  frequency 
of  the  westerly  winds  increases  to  nearly  twice  that  at  the  lower 
level,  and  there  is  a  corresponding  increase  in  velocity.  Particular 
attention  is  paid  to  the  problem  of  trans- Atlantic  flight  and  the 
possibility  of  utilizing  the  northeast  trade  for  the  western  voyage  is 
considered.  The  height  at  which  the  southwest  counter  trade  may 
be  expected  is  uncertain.  It  has  sometimes  been  found  below  1,500 
meters,  and  again  has  been  absent  at  10,000  meters. 

Another  excellent  book  is  that  of  C.  J.  P.  Cave,  entitled  "The 
Structure  of  the  Atmosphere  is  Clear  Weather,"  which  gives  the 
result  of  200  observations  of  pilot  balloons  and  "baflons-sondes." 
The  direction  and  velocity  of  the  wind  at  different  levels  are  charted. 
Cardboard  models  show  the  distribution  of  wind  with  each  kilometer 

1  Charts  of  the  Atmosphere  for  Aeronauts  and  Aviators. 
50821—18 3 


34  MANUAL   OF  AEROGBAPHY. 

of  height.  The  arrowhead  flies  with  the  wind.  The  gradient  wind  is 
computed  from  the  distribution  of  pressure.  The  velocity  is  cal- 
culated from  the  measured  distance  of  two  isobars  between  which 
the  station  lies  by  means  of  the  formula. 

Gp  =  2pco  v  sin  0 

where  Gp  is  the  gradient,  co  the  angular  velocity  of  the  earth,  <£  the 
latitude,  v  the  velocity,  and  p  the  density  of  the  air. 

Five  types  of  atmospheric  structure  are  described:  (1)  Where  the 
wind  in  the  upper  air  is  steady  and  there  is  no  increase  of  velocity 
with  height;  (2)  where  the  wind  is  steady,  but  increases  in  velocity 
much  above  the  gradient  value;  (3)  where  the  upper  wind  decreases 
in  velocity;  (4)  where  changes  of  direction  or  reversals  occur;  and 
(5)  where  the  upper  air  blows  away  from  centers  of  low  pressure. 

The  strongest  current  is,  as  a  rule,  just  below  the  stratosphere; 
and  in  view  of  the  work  of  Dines  and  the  suggestions  of  Shaw,  the 
question  is  raised  whether  it  would  not  be  advantageous  to  refer 
variations  in  the  different  levels  to  the  conditions  in  the  9-kilometer 
level  instead  of  to  the  surface.  Starting  with  a  strong  westerly 
wind  under  the  stratosphere,  Cave  would  then  work  downward,  for 
almost  without  exception  the  west  wind  decreases  in  the  lower 
levels,  and  the  falling  off  may  proceed  continuously  to  such  an  extent 
that  the  direction  of  motion  is  reversed  at  some  point  in  the  inter- 
mediate layers,  so  that  near  the  surface  an  easterly  wind  is  shown 
instead  of  the  westerly  one  of  the  upper  levels. 

The  term  " holes  in  the  air"  has  been  used  by  aviators  to  describe 
certain  unstable  conditions  experienced  when  flying  and  which  in 
their  opinion  are  caused  either  by  " holes"  in  the  air,  or  by  partial 
vacuums  or  "  pockets."  Such  conditions  are  found  on  summer 
mornings  and  afternoons  and  near  cumulus  clouds,  and  are  gener- 
ally simply  ascending  currents  of  some  momentum  through  which 
the  flyer  passes.  Sometimes  the  aviator  may  skirt  such  a  column 
of  uprising  air  (and  there  are  also  descending  currents),  and  part  of 
the  plane  be  within  the  current,  while  the  rest  may  be  without.  In 
such  cases  there  will  be  sudden  tilting  or  inequality  of  pressure,  and 
the  aviator  should  be  careful  not  to  attempt  to  meet  the  changed 
conditions  too  quickly,  for  they  are  but  temporary  and  instability 
will  result  when  the  machine  is  again  free.  Not  only  near  cumuli, 
also  near  cross  currents — that  is,  where  one  air  stream  is  flowing  in  a 
different  direction  from  an  adjacent  stream — there  will  be  minor 
vortices  and  more  or  less  of  an  air  surge;  and  such  a  condition  will 
cause  instability  of  the  airplane.  As  has  been  explained  in  pre- 
ceding paragraphs,  there  is  sometimes  a  marked  stratification  of  the 
lower  air,  and  under  certain  conditions  marked  turbulence.  When 
such  conditions  are  suspected  it  might  be  advisable  to  resort  to  pre- 
liminary tests  by  freezing  pilot  balloons  or  pilot  planes. 

A.  McA. 


MANUAL  OF  AEROGRAPHY. 


35 


TABLES  FOR  THE  COMPUTATION  OF  HUMIDITY  FROM  THE  READINGS  OF  THE  DRY 
AND  WET  THERMOMETERS  ox  THE  KELVIN  SCALE  OF  ABSOLUTE  TEMPERATURE 
BASED  ON  GLAISHER'S  FACTORS. 

(From  Computer's  Handbook,  Meteorological  Office.] 


Per  cent 

Dry  thermometer. 

ofrela- 

t  ivo  hu- 

midity. 

270" 

271° 

272° 

273° 

274° 

275° 

276° 

277° 

278° 

279° 

WKT  THERMOMETER. 

0 

0 

0 

0 

0 

0 

0 

0 

99 

270.0 

271.0 

272.0 

273.0         274.0         274.9         275.9 

276.9         277.9 

278.9 

269.9 

270.  9 

271.8 

272.8         273.7          274.7         275.7 

276.  7         277.  7 

278.7 

90 

269.8 

270.7 

271.6 

272.6         273.5          274.4         275.4 

276.4         277.3 

278.3 

x.-, 

ttO.fl 

270.6 

271.5  ,      272.3         273.2          274.1         275.1 

276.  0         277.  0 

277.9 

80 

269.5 

270.4 

271.3 

272.1         272.9          273.8         274.7 

275.6 

276.6 

277.5 

75 

269.3 

270.2 

271.1 

271.8        272.6  ,       273.4 

274.3 

275.3 

276.2 

277.1 

70 

269.2 

270.0 

270.8 

271.5         272.3          273.1         273.9         274.8         275.8 

276.7 

65 

269.0 

269.8 

270.6 

271.2         271.9          272.7         273.5         274.4         275.3 

276.2 

60 

268.8 

269.6 

270.3 

270.9         271.5          272.2         273.1         273.9         274.8 

275.7 

55 

268.6 

269.4 

270.0 

270.6         271.1          271.8 

272.6 

273.  4        274.  3 

275.2 

50 

_>'iv  4 

269.2 

269.7 

270.2        270.7          271.3 

272.1 

272.  9         273.  8 

274.6 

45 

268.2 

268.9 

269.4 

269.8         270.2          270.7         271.5         272.3 

273.1 

274.0 

40 

268.0 

268.6 

269.0 

269.4         269.7          270.2         270.  S         271.6 

272.5 

273.  3 

35 

267.7 

268.3 

268.6 

268.9         269.1          269.5         270.1 

270.9 

271.7 

272.5 

30 

267.4 

267.9 

268.2 

268.3         268.4          268.8         269.3        270.0        270.8 

271.6 

25 

267.0 

267.5 

267.7 

267.6         267.6          267.9         268.4         269.1 

269.8 

270.5 

20 

266.6 

266.9 

267.0 

266.  9         266.  7          266.  8  ;      267.  3 

267.9 

268.6 

269.3 

10 

265.3 

265.4 

265.2 

264.  5         263.  9          263.  7  j      264.  0 

264.5 

265.1 

265.6 

VAPOR  PRESSURE. 

Kb. 

Kb. 

Kb. 

Kb. 

Kb.           Kb. 

Kb. 

Kb. 

Kb. 

Kb. 

100 

4.89 

5.27 

5.66 

6.09 

6.  54            7.  03           7.  55 

8.09 

8-68 

9.29 

90 

4.  40           4.  74 

5.09           5.48 

5.89           6.33           6.80 

7.28 

7.81. 

8.36 

80 

3.91           4.22           4.53           4.87 

5.23            5.62          6.04 

6.47 

6.94 

7.43 

70 

3.42          3.69           3.96          4.26 

4.58  i         4.92          5.29 

5.66 

6.08 

6.50 

60 

2.93           3.16           3.40          3.65 

3.  92           4.  22           4.  53 

4.85 

5.21 

5.57 

50 

2.  45           2.  64           2.  83           3.  05 

3.27 

3.52           3.78 

4.05 

4.34 

4.65 

40 

1.96           2-11           2.26           2.24 

2.62 

281           3.02 

3.24 

3.47 

3.72 

30 

1.47           1.58           1.70           1.83 

1.96           2.11           2.27 

2.43 

2.60 

2.79 

20 

0.98           1.05           1.13           1.22 

1.31            1.41           1.51 

1.62 

1.73 

1.86 

10 

0.49          0.53           0.57.        0.61 

0.65 

0.70          0.76 

0.81 

0.87 

0.93 

Per  cent 

nt  VAln 

Dry  thermometer. 

oi  rela- 
tive hu- 
midity. 

280° 

281° 

282°           283° 

284°            285° 

286° 

287° 

288° 

289° 

WET  THERMOMETER. 

0                                  0 

0 

, 

i 

o 

0 

0 

0 

0 

99 

279.  9         280.  9 

281.9 

282.9 

283.9 

1M.  11 

285.9 

286.9 

287.9  I        288.9 

95 

279.7 

280.6 

281.6 

282.6 

283.6 

284.6 

285.6 

2Mi.  n 

287.6           288.6 

90 

279.3 

280.3 

281.3 

282.2 

283.2 

284.2 

285.2 

286.2 

287.  1           288.  1 

85 

278.9 

279.9 

280-  fl 

281.8 

282.8 

283.8 

284.7 

285.7 

286.  7           287.  7 

80 

278.5 

279.5 

280.4 

281.4 

282.4 

283.3 

284.3 

285.2 

286.  2           287.  2 

75 

878,  1 

279.0 

280.0 

280.9 

281.9 

282.8 

283.8 

284.7 

285.  7  '        286.  6 

70 

277.6 

278.6 

279.5 

281.4 

2M>.3 

283.3 

284.2 

285.  1  ;        286.  1 

65 

277.2 

278.1 

279.0 

27'.).  !» 

280.8 

281.8 

282.7 

283.6 

284.5          285.5 

60 

276.7 

277.6 

278.5 

279.4 

280.3 

281.2 

282.1 

283.0 

283.  9  •        284.  8 

55 

276.1 

277.0 

277.9 

278.8 

279.7 

280.6 

281.5 

282.3 

283.2          284.2 

50 

275.5 

276.4 

277.  3 

27X.  2 

279.0 

279.9 

MO.  8 

281.6 

282.5  ;        283.4" 

1.5 

274.8 

275.7 

276.6 

277..-, 

278.3 

279.2 

280.0 

280.9 

281.7  •        282.6 

40 

274.1 

275.0 

27'..  x 

276.7 

277.  5 

278.3 

279.2 

280.0 

280.  8  ;        281.  7 

35 

273.3 

274.1 

275.  X 

276.6 

277.4 

278.  2 

279.0 

279.  9           280.  7 

30 

272.4 

273.2 

274.0 

274.  S 

275.6 

276.4 

277.2 

277.9 

278.7 

279.6 

25 

271.3         272.1 

272.9 

273.7 

274.4 

275.2 

275.  9 

276.7 

277.4           278.2 

20 

270.  0         270.  8 

271.6 

272.3 

273.0 

273.7 

274.4 

275.2 

275.  9           276.  7 

10 

266.4         267.0 

267.6 

na  •» 

268.9 

269.5 

270.1 

270.7 

271.3           272.0 

36 


MANUAL   OF   AEKOGRAPHY. 


1.  TABLES  FOR  THE  COMPUTATION  .OF  HUMIDITY  FROM  THE  READINGS  OF  THE  DRY 
AND  WET  THERMOMETERS  ON  THE  KELVIN  SCALE  OF  ABSOLUTE  TEMPERATURE 
BASED  ON  GLAISHER'S  FACTORS — Continued. 


Per  cent  1                                                                Drv  thermometer. 

of  rela- 

tive hu- 

j 

midity. 

280°      ;     281° 

282° 

283° 

284° 

285° 

286°      |     287° 

288° 

289° 

VAPOR  PRESSURE. 

Kb. 

Kb. 

Kb. 

Kb. 

Kb. 

Kb. 

Kb. 

Kb. 

Kb. 

Kb. 

100 

9.96 

10.65 

11.40 

12.19 

13.02 

13.91 

14.85 

15.84 

16.89 

18.01 

90 

8.96 

9.59 

10.26 

10.97 

11.72 

12.52 

13.37 

14.26 

15.20 

16.21 

80 

7.97 

8.52 

9.12 

9.75 

10,  42 

11.13         11.88 

12.67 

13.51 

14.41 

70 

6.97 

7.46 

7.98 

'8.53 

9.11 

9.  74         10.  40 

11.09 

11.82 

12.61 

60 

5.98 

6.39 

6.84 

7.31 

7.81 

8.35           8.91 

9.50 

10.13 

10.81 

50           4.  98 

5.33 

5.70 

6.10 

6.51 

6.96 

7.43 

7.92 

^6.45 

9.01 

40           3.98 

4.26 

4.56 

4.88 

5.21 

5.  56 

5.94 

6.34 

6.76 

7.20 

30           2.  99 

3.20 

3.42 

3.66 

3.91 

4.17 

4.46 

4.75 

5.07 

5.40 

20           1.99 

2.13 

2.28 

2.44 

2.60 

2.78 

2.97 

3.17 

3.38  i          3.60 

10           1.00 

1.07 

1.14 

1.22 

1.30 

1.39 

1.49 

1.58 

-•  1.69 

1.80 

DRY  THERMOMETER. 


Per  cent 
of  rela- 
tive hu- 
midity. 

290° 

291° 

292° 

293° 

294° 

295° 

296° 

297° 

298° 

299° 

99        

WET  THERMOMETER. 

289.9 
289.6 
289.1 

288.6 
288.1 

287.6 
287.  0- 
286.4 
285.8 
285.1 

284.3 
283.5 
282.6 
281.6 
280.4 

279.1 
277.4 
272.7 

290.9 
290.6 
290.1 
289.6 
289.1 

288.5 
288.0 
287.3 
286.7 
286.0 

285.2 

284.4 
283.4 
282.4 
281.2 

279.8 
278.2 
273.4 

291.9 
291.6 
291.1 
290.6 
290.1 

289.5 
288.9 
288.3 
287.6 
286.9 

286.1 
285.2 
284.3 
283.2 
282.0 

280.6 
279.0 
274.1 

292.9 
292.5 
292.1 
291.5 
291.0 

290.5 

289.8 
289.2 

288.5 
287.8 

287.0 
286.1 
285.1 
284.1 
282.  9 

281.4 
279.7 
274.8 

293.9 
293.5 
293.0 
292.5 
292.0 

291.4 

290.8 
290.1 
289.4 
288.7 

287.9 
287.0 
286.0 
284.9 
283.7 

282.2 
280.5 
275.4 

294.9 
294.5 
294.0 
293.5 
292.9 

292.3 
291.7 
291.1 
290.4 
289.6 

288.8 
287.9 
286.9 
285.7 
284.5 

283.0 
281.2 
276.1 

0 

295.9 
295.5 
295.0 
294.5 
293.9 

293.3 

292.7 
292.0 
291.3 
290.5 

289.7 
288.8 
287.7 
286.6 
285.3 

283.8 
282.1 
276.8 

296.9 
296.5 
296.0 
295.4 
294.9 

294.3 
293.6 
292.9 
292.2 
291.4 

290.6 
289.6 
288.6 
287.4 
286.1 

284.6 
282.8 
277.4 

297.9 

297.5 
297.0 
296.4 
295.8 

295.2 
294.6 
293.9 
293.1 
292.3 

291.4 
290.5 
289.4 
288.3 
286.9 

285.4 
283.5 
278.1 

298.9 
298.5 
298.0 
297.4 
296.8 

296.2 
295.5 
294.8 
294.1 
293.2 

292.4 
291.4 
290.3 
289.1 
287.8 

286.2 
284.4 
278.8 

95 

90      

85 

80  

75  

70 

65 

60 

55 

50 

45     

40 

35     

30 

25  
20  

10 

100... 
90 

VAPOR  PRESSURE. 

Kb. 
19.20 
17.28 
15.36 
13.44 
11.52 

9.60 
7.68 
5.76 
3.84 
1.92 

Kb. 
20.44 
18.40 
16.35 
14.31 
12.26 

10.22 
8.18 
6.13 
4.09 
2.04 

Kb. 
21.76 
19.58 
17.42 
15.  23 
13.07 

10.88 
8.70 
6.53 
4.35 
2.18 

Kb. 
23.14 
20.83 
18.51 
16.20 
13.88 

11.57 

9.26 
6.94 
4.63 
2.31 

Kb. 
24.62 
22.16 
19.70 
17.23 
14.77 

12.31 
9.85 
7.39 
4.92 
2.46 

Kb. 
26.17 
23.55 
20.94 
18.32 
15.70 

13.09 
10.47 
•7.85 
5.23 
2.62 

Kb. 
27.81 
25.03 
22.25 
19.47 
16.69 

13.91 
11.12 
8.34 
5.56 
2.78 

•     Kb. 
29.  53 
26.58 
23.62 
20.67 
17.72 

14.77 
11.81 
8.86 
5.91 
2.95 

Kb. 
31.36 

28.22 
25.09 
21.95 
18.82 

15.68 
12.54 
9.41 
6.27 
3.14 

Kb. 
33.28 
29.95 
26.62 
23.30 
19.97 

16.64 
13.31 
9.99 
6.66 
3.33 

80 

70        

60 

50 

40  

30  

20 

10  

MANUAL   OF  AEROGRAPHY. 


37 


1.  TABLES  FOR  THE  < OMIM  TATIOX  OF  HUMIDITY  FROM  THE  READINGS  OF  THE  DRY 

AM>    \\"KT    TllKltM'iMKTERS    ON    THE    KELVIN    SCALE    OP    ABSOLUTE    TEMPERATURE 

BASED  ox  (JI.AISHEK'S  FACTORS — Continued. 


Dry  thermometer. 

Per  cent 
of  rela- 
tive hu- 

300° 

301° 

302° 

303° 

304° 

305° 

306° 

307° 

308° 

309° 

midity. 

Wet  thermometef  . 

99 

299.0 

300.9 

O 

301.9 

302.9 

303.9 

304.9        305.9 

O 

306.9 

307.9 

308.9 

95 

299.5 

300.5 

301.5 

302.5 

303.5 

304.  5         305.  4         306.  4 

307.4 

308.4 

90 

298.9 

299.9 

300.9 

301.9 

302.9          303.9         304.9         305.8 

306.8 

307.8 

85 

298.4 

299.4 

300.3 

301.3 

302.3          303.3         304.2         305.2 

306.2 

307.2 

80 

297.8 

298.7 

299.7 

300.7 

301.7 

302.  6         303.  6         304.  6 

305.5 

306.5 

75 

297.1 

298.1 

299.1 

300.0 

301.0 

301.  9        302.  9        303.  9 

304.8 

305.8 

70 

296.5 

297.4 

298.4 

299.3 

300.3          301.2         302.2         303.1 

304.1 

305.0 

65 

295.8 

296.7 

297.  6         298.  6 

299.5  i       300.5         301.4         302.3 

303.3 

304.2 

60 

295.0 

295.9 

296.  8         297.  7 

298.7          299.6         300.6 

301.5 

302.4 

303.3 

H 

294.2 

295.1 

296.0 

296.9 

297.8 

298.  7        299.  7        300.  6 

301.5 

302.4 

50 

293.3 

294.2 

295.1 

296.0 

296.9 

297.  8        298.  7 

299.6" 

300.5 

301.4 

45 

292.3 

293.2 

294.1 

295.0 

295.9 

296.8         297.7  .      298.5 

299.4 

300.3 

40 

291.2 

292.1 

293.0 

293.9 

294.8 

295.  6         296.  5         297.  4 

298.2 

299.1 

35 

290.0 

290.9 

291.7 

292.6 

293.5 

294.  3         295.  2        296.  1 

296.9 

297.8 

30 

288.6 

289.5 

290.3 

291.2 

292.0 

292.  9        293.  7         294.  6 

295.4 

296.2 

25 

287.0 

287.9 

288.7 

289.5 

290.4 

291.  2         292.  0         292.  8 

293.6 

294.5 

20 

285.2 

286.0 

286.8 

287.6 

288.4 

289.  2         290.  0        290.  8 

291.5 

292.3 

10 

279.6 

280.3 

281.0 

281.7 

282.4 

283.2 

283.9        284.6 

285.4 

286.1 

Vapor  pressure. 

Kb. 

Kb. 

Kb. 

Kb. 

Kb. 

Kb. 

Kb. 

Kb. 

Kb. 

Kb. 

100 

35.29 

37.42 

39.65 

42.01 

44.49 

47.09 

49.83 

52.69 

55.71 

58.88 

90 

31.  76        33.  68 

35.69 

37.81 

40.04 

42.38 

44.85 

47.42 

50.  14          52.  99 

80 

28.23        29.94 

31.72 

33.61 

35.59 

37.67 

39.86 

42.15 

44.  57           47.  10 

70 

24.  70        26.  19 

27.76 

29.41 

31.14 

32.96 

34.88 

36.88        39.00          41.22 

60 

21.17         22,45 

23.79 

25.21 

26.69 

28.25 

29.90 

31.61  [      33.43 

35.33 

50 

17.65 

18.71 

19.83 

21.01 

22.  25          23.  55 

24.92 

26.35 

27.86 

29.44 

40 

14.  12         14.  97 

15.86 

16.80 

17.  80         18.  84 

19.93 

21.08 

22.28 

23.55 

30 

10.59         11.23 

11.90 

12.60 

13.  35          14.  13 

14.95 

15.81 

16.71 

17.66 

20 

7.  06          7.  48 

7.93 

8.40 

8.90           9.42 

9.97 

10.54 

11.14 

11.78 

10 

3.53 

3.74 

3.97 

4.20 

4.45 

4.71 

4.98 

5.27 

5.57 

5.89 

A.  McA. 


CHAPTER  IV. 


HOLES  IN  THE  AIR. 


39 


CHAPTER   IV. 


"HOLES   IN   THE   AIR." 

By  PROF.  \V.  J.  HUMPHREYS. 

General  statement. — Every  aviator  experiences  in  the  course 
of  his  flights  many  seemingly  abrupt  drops,  and  numerous  more  or 
less  severe  jolts.  The  causes  of  the  first — the  sudden  drops — he  has 
grouped  together  and  called  "  holes  in  the  air/'  while  to  the  latter 
he  has  given  such  names  as  "  bumps,"  "dunts,"  etc.  There  are,  of 
course,  no  holes,  in  the  ordinary  sense  of  the  term,  in  the  atmosphere — 
no  vacuous  regions — but  the  phrase  "holes  in  the  air"  graphically 
expresses  the  fact  that  occasionally,  at  various  places  in  the  atmos- 
phere, there  are  conditions  which,  so  far  as  flying  is  concerned,  are 
very  like  unto  holes.  Such  conditions  are  indeed  real  and,  because 
of  both  their  general  interest  and  practical  importance,  deserving  of 
careful  study. 

Before  taking  up  these  actual  conditions,  however,  it  will  be  con- 
venient first  to  show  that  no  matter  how  suddenly  an  aviator  may 
drop,  it  can  not  be  because  he  has  run  into  a  vacuum.  Suppose  for 
a  moment  that  there  was  a  great  hole  in  the  quiet  atmosphere,  in  the 
sense  of  a  place  devoid  of  air.  Obviously  the  surrounding  atmos- 
phere would  begin  rushing  into  this  space  with  the  velocity  given 
by  the  equation,  v  =  ^/2gH,  in  which  g  is  gravity  acceleration,  and 
H  the  virtual  height  of  the  atmosphere,  approximately  8  kilometers. 
Hence,  since  v  equals  400  meters  per  second,  roughly,  or  neatly  900 
miles  per  houi,  even  if  such  a  hole  existed  it  would  be  impossible 
for  an  aviator  to  get  into  it — he  could  not  catch  up  with  its  closing 
walls.  But,  according  to  the  claims  of  some,  if  there  are  no  com- 
plete holes  in  the  atmosphere,  there  are,  at  any  rate,  places  where 
the  density  is  much  less  than  that  of  the  surrounding  air;  so  much 
less,  indeed,  that  when  an  aeroplane  runs  into  one  of  them  it  drops 
quite  as  though  it  were  in  a  place  devoid  of  all  air  and  without  sup- 
port of  any  kind. 

This,  too,  like  the  actual  hole,  is  a  pure  fiction  that  has  no  warrant 
in  barometric  records.  Indeed,  such  a  condition  could  be  estab- 
lished and  maintained  only  by  a  gyration  or  whirl  of  the  atmosphere, 
such  that  the  centrifugal  force  would  be  sufficient  to  equal  the 
difference  in  pressure,  at  the  same  level,  between  the  regions  of  high 
and  low  density. 

Let,  for  instance,  the  pressure  gradient  be  uniform  about  a  cir- 
cular area  and  so  great  that  the  barometer  reading  varies 
from  0  at  the  center  to  1013  Kb.  at  the  radial  distance  0.5  km. 

41 


42  .         MANUAL  OF   AEBOGRAPHY. 

On   substituting   these    values   in   the   gradient  velocity  equation, 

-    _             .           mv2  ,      ,      /.    mv2        ,  v     •       Ai 

j  =  2mco  v  sin  <H >  or,  very  closely,  /=  - — »  and  rememberuig  that 

the  density  varies  as  the  pressure,  or,  in  this  case,  as  r,  that  is  from 
0  where  r  =  0  to  .00129  where  r  =  0.5  kilometers,  it  appears  that  the 
linear  velocity  of  the  wind  in  the  tornadic  whirl  that  would  sustain 
such  a  gradient  would  be  the  same  throughout,  and  approximately 
280  meters  per  second  (626  miles  per  hour).  But  such  large  and 
violent  whirls  are  unknown;  even  tornadoes  seldom  reduce  the 
pressure  more  than  10  per  cent.  Hence  neither  vacua  nor  even 
isolated  and  dangerous  partial  vacua  occur  in  the  atmosphere. 

Along  with  these  two  impossibles,  the  vacuum  and  the  half  vacuum, 
should  be  consigned  to  oblivion  that  other  picturesque  fiction,  the 
"pocket  of  noxious  gas/'  Probably  no  other  gases,  certainly  very 
few,  have,  at  ordinary  temperatures  and  pressures,  the  same  density 
as  atmospheric  air.  Therefore,  a  pocket  of  foreign  gas  in  the  atmos- 
phere would  almost  certainly  either  bob  up  like  a  balloon,  or  sink 
like  a  stone  in  water;  it  could  not  float  in  mid-air.  It  is  possible,  of 
course,  as  will  be  explained  a  little  later,  to  run  into  columns  of  rising 
air  that  may  contain  objectional  gases  and  odors,  but  these  columns 
are  quite  different  from  anything  likely  to  be  suggested  by  the 
expression  "pocket  of  gas." 

The  above  are  some  of  the  things  that,  fortunately,  do  not  exist. 
The  following,  however,  are  some  that  do  exist  and  produce  effects 
such  as  actual  vacua  and  partial  vacua  would  produce — sudden 
drops,  usually  small  but  occasionally  very  considerable,  and,  though 
rarely,  even  disastrous  falls. 

Air  fountains. — A  mass  of  air  rises  or  falls  according  as  its  density 
is  less  or  greater,  respectively,  than  that  of  the  surrounding  atmos- 
phere, just  as  and  for  the  same  reason  that  a  cork  bobs  up  in  water 
and  a  stone  goes  down. 

Hence  warm  and  therefore  expanded  and  relatively  light  air  is 
driven  up  whenever  the  surrounding  air  at  the  same  level  is  colder, 
and  as  the  atmosphere  is  heated  mainly  through  contact  with  the 
surface  of  the  earth,  which  in  turn  has  been  heated  by  sunshine,  it 
follows  that  these  convection  currents  or  vertical  uprushes  of  the 
atmosphere,  are  most  numerous  during  calm  summer  afternoons. 

The  turbulence  of  some  of  these  rising  masses  is  evident  from  the 
numerous  rolls  and  billows  of  the  large  cumulus  clouds  they  produce, 
and  it  is  obvious  that  the  same  sort  of  turbulence,  probably  on  a 
smaller  scale,  occurs  near  the  top  of  such  columns  as  do  not  rise  to 
the  cloud  level.  Further,  it  is  quite  possible,  when  the  air  is  excep- 
tionally quiet,  for  a  rising  column  to  be  rather  sharply  separated  from 
the  surrounding  quiescent  atmosphere,  as  is  evident  from  the  closely 
adhering  long  pillars  of  smoke  occasionally  seen  to  rise  from  chimneys. 


MANUAL   OF   AEROGRAPHY.  43 

The  velocity  of  ascent  of  such  fountains  of  air  is  at  times  surpris- 
ing! v  irreat.  Measurements  on  pilot  balloons  and  also  measurements 
taken  in  manned  balloons  have  shown  vertical  velocities,  both  up 
and  down,  of  more  than  3  meters  per  second.  The  soaring  of  large 
birds  is  a  further  proof  of  an  upward  velocity  of  the  same  order  of 
magnitude,  while  the  fact  that  in  cumulus  clouds  water  drops  and 
hailstones  often  are  not  only  temporarily  supported,  but  even  carried 
to  higher  levels,  shows  that  uprushes  of  at  least  8  to  10  or  12  meters 
per  second  not  merely  may  but  actually  do  occur. 

There  are,  then,  air  fountains  of  considerable  velocity  whose  sides 
at  times  and  places  may  be  almost  as  sharply  separated  from  the 
surrounding  atmosphere  as  are  the  sides  of  a  fountain  of  water,  and 
it  is  altogether  possible  for  the  swiftest  of  these  to  produce  effects 
on  an  aeroplane  more  or  less  disconcerting  to  the  pilot.  The  trouble 
may  occur: 

1.  On  grazing  the  column,  with  one  wing  of  the  machine  in  the 
rising  and  the  other  in  the  nonrising  air;  a  condition  that  interferes 
with  the  lateral  stability  and  produces   a  sudden  shock  both  on 
entering  the  column  and  on  leaving  it. 

2.  On  plunging  squarely  into  the  column;  thus  suddenly  increasing 
the  angle  of  attack,  the  pressure  on  the  wings,  and  the  angle  of  ascent. 

3.  On   abruptly  emerging  from  the  column;  thereby  causing  a 
sudden   decrease  in  the  angle  of  attack   and  also   abruptly  losing 
the  supporting  force  of  the  rising  mass  of  air. 

4.  As  a  result  of  rotation,  if  rapid,  as  it  sometimes  is,  of  the  rising 
air. 

That  flying  with  one  wing  in  the  column  and  the  other  out  must 
interfere  with  lateral  stability  and  possibly  cause  a  drop  as  though 
a  hole  had  been  encountered,  is  obvious,  but  the  effects  of  plunging 
squarely  into  or  out  of  the  column  require  a  little  further  consideration 
as  does  also  the  effect  of  rotation. 

Let  an  aeroplane  that  is  flying  horizontally  pass  from  quiescent 
air  squarely  into  a  rising  column.  The  front  of  the  machine  will  be 
lifted,  as  it  enters  the  column,  a  little  faster  than  the  rear,  and  the 
angle  of  attack,  that  is,  the  angle  which  the  plane  of  the  wing,  or  plane 
of  the  wing  chords,  makes  with  the  apparent  wind  direction,  will  be 
slightly  increased.  This,  together  with  the  rising  air,  will  rapidly 
carry  the  machine  to  higher  levels,  which,  of  itself,  is  not  important. 
If,  however,  the  angle  of  attack  is  so  changed  by  the  pilot  as  to  keep 
the  machine,  while  in  the  rising  column,  at  a  constant  level,  and  if, 
with  this  new  adjustment  the  rising  column  is  abruptly  left,  a  rapid 
descent  must  begin.  But  even  this  is  not  necessarily  harmful. 

Probably  the  real  danger  under  such  circumstances  arises  from 
over  adjustments  by  the  aviator  in  his  hasty  attempt  to  correct  for 
the  abrupt  changes.  Such  an  adjustment  might  well  cause  a  fall  so 
sudden  as  strongly  to  suggest  an  actual  hole  in  the  air. 


44  MANUAL  OF   AEKOGRAPHY. 

If  the  rising  column  is  in  fairly  rapid  rotation  (tornadoes  are 
excluded — they  can  be  seen  and  must  be  avoided),  as  sometimes  is 
the  case,  disturbances  may  be  produced  in  several  ways.  If  the 
column  is  entered  on  its  approaching  side,  the  head-on  wind  may  so 
decrease  the  velocity  of  the  plane  with  reference  to  the  surrounding 
air  that  on  emerging  there  necessarily  must  be  a  greater  or  less  drop; 
as  explained  below  under  the  caption  "  Wind  layers."  On  the  other 
hand,  if  entered  on  the  receding  side  there  will  be  a  tendency  to  drop 
within  the  column,  which  may  or  may  not  be  fully  compensated 
by  the  vertical  component  of  the  wind.  Finally,  such  a  rotating 
column,  especially,  perhaps,  if  crossed  near  its  outer  boundary,  may 
quickly  change  the  orientation  of  the  plane  and  therefore  the  action 
on  it  of  the  surrounding  air. 

None  of  these  conditions,  however,  except  when  encountered  near 
the  surface  of  the  earth  is  likely  to  involve  any  appreciable  element 
of  danger  to  the  skillful  aviator.  But  this  does  not  justify  ignoring 
them — no  beginner  is  skillful,  and  all  must  start  from  and  return  to 
the  surface. 

Rising  columns  of  the  nature  just  described  occur  most  frequently 
during  clear  summer  days  and  over  barren  ground.  They  also  occur, 
even  to  surprising  altitudes,  over  roads,  sandpits,  and  other  places 
of  similar  contrast  to  the  surrounding  areas.  Isolated  hills,  especially 
short  or  conical  ones,  should  be  avoided  during  warm  still  days,  for 
on  such  occasions  their  sides  are  certain  to  be  warmer  than  the 
adjacent  atmosphere  at  the  same  level,  and  hence  to  act  like  so  many 
chimneys  in  producing  updrafts.  Rising  air  columns  occur  less 
frequently  and  are  less  vigorous  over  water  and  over  level  green 
vegetation  than  elsewhere.  They  are  also  less  frequent  during  the 
early  forenoon  than  in  the  hotter  portion  of  the  day,  and  are  prac- 
tically absent  before  sunrise  and  at  such  times  as  the  sky  is  wholly 
covered  with  clouds. 

Air  cataracts. — There  are  two  kinds  of  air  cataracts,  the  free  air 
cataracts  and  the  surface  cataract.  The  former  is  the  counterpart 
of  the  air  fountain  and  is  most  likely  to  occur  at  the  same  time. 

Indeed  it  is  certain  to  occur  over  a  small  pond,  lake,  or  clump  of 
trees  in  the  midst  of  a  hot  and  rather  barren  region.  These  cooler 
spots  localize  the  return  or  down  branches  of  the  convection  currents, 
and  generally  should  be  avoided  by  the  aviator  when  flying  at  low 
levels.  Similarly,  on  calm,  clear,  summer  days  down  currents 
nearly  always  obtain  at  short  distances  offshore,  over  rivers,  and 
along  the  edges  of  forests.  The  free-air  cataract,  however,  seldom 
is  swift,  except  in  connection  with  thunderstorms,  and,  therefore 
while  it  may  render  flying  difficult  or  even  impossible  with  a  slow 
machine  it  seldom  involves  much  danger. 


.MAXt'AL   OF    AEROGRAPH Y.  45 

The  second,  or  surface  cataract,  is  caused  by  the  flow  of  a  dense  or, 
what  comes  to  the  same  thing,  a  heavily  laden  surface  layer  of  air 
up  to  and  then  over  a  precipice,  much  as  a  waterfall  is  formed. 
Such  cataracts  are  most  frequent  among  the  barren  mountains  of 
high  latitudes  where  the  cold  surface  winds  catch  up  and  become 
weigh  tod  with  great  quantities  of  dry  snow  and,  because  of  both 
this  extra  weight  and  their  high  density  often  rush  down  the  lee 
sides  of  steep  mountains  with  the  roar  and  force  of  a  hurricane. 
But  the  violence  of  such  winds  clearly  is  all  on  the  lee  side  and  of 
shallow  depth.  Hence,  where  such  conditions  prevail  the  aviator 
should  keep  well  above  the  drifting  snow  or  other  aerial  ballast,  and,  if 
possible,  strictly  avoid  any  attempt  to  land  withiiuthe  cataract  itself. 

Cloud  currents. — It  frequently  happens  that  a  stratum  of 
broken  or  detached  clouds,  especially  of  the  cumulus  type,  is  a  region 
of  turbulent  currents,  however  quiet  the  air  at  both  lower  and  higher 
levels.  In  the  case  of  cumuli,  at  least,  the  currents  within  the  clouds 
are  upward,  and  those  in  the  open  spaces,  therefore,  in  general, 
downward,  with,  of  course,  in  each  case  greater  or  less  turbulence. 
Hence  while  passing  through  such  a  layer  the  aviator  is  likely  to 
encounter  comparatively  rough  flying,  though,  owing  to  the  height, 
of  very  little  danger. 

Air  cascades. — The  term  "air  cascades"  may,  with  some  pro- 
priety, be  applied  to  the  wind  as  it  sweeps  down  the  lee  of  a  hill  or 
mountain.  Ordinarily  it  does  not  come  very  near  the  ground,  where 
indeed  there  frequently  is  a  counter  current,  but  remains  at  a  con- 
siderable elevation.  Other  things  being  equal,  it  is  always  most 
pronounced  when  the  wind  is  at  right  angles  to  the  direction  of  the 
ridge  and  when  the  mountain  is  rather  high  and  steep.  The  swift 
downward  sweep  of  the  air  .when  the  wind  is  strong  may  carry  a 
passing  aeroplane  with  it  and  lead  observers,  if  not  the  pilot,  to 
fancy  that  a  hole  has  been  encountered  where,  of  course,  there  is 
nothing  of  the  kind.  Indeed,  such  cascades  should  be  entirely  harm- 
loss  as  long  as  the  aviator  keeps  his  machine  well  above  the  surface 
and  therefore  out  of  the  treacherous  eddies,  presently  to  be  discussed. 

Wind  layers. — For  one  reason  or  another  it  often  happens  that 
adjacent  layers  of  air  differ  abruptly  from  each  other  in  temperature, 
humidity,  and  density,  and,  therefore,  as  explained  by  Helmholtz, 
may  and  often  do  glide  over  each  other  in  much  the  same  manner  that 
air  flows  over  water  and  with  the  same  general  wave-producing 
effect.  These  air  waves  are  "seen"  only  when  the  humidity  at  the 
interface  is  such  that  the  slight  difference  in  temperature  between 
the  crests  and  the  troughs  is  sufficient  to  keep  the  one  cloud-capped 
and  the  other  free  from  condensation.  In  short,  the  humidity  con- 
dition must  be  kept  just  right;  clearly,  then,  though  such  clouds 


46  MANUAL  OF   AEROGRAPHY. 

often  occur  in  beautiful  parallel  rows,  adjacent  wind  strata  of  different 
velocities  and  their  consequent  air  billows  must  be  of  far  more  fre- 
quent occurrence. 

Consider  now  the  effect  on  an  aeroplane  as  it  passes  from  one  such 
layer  into  another.  For  the  sake  of  illustration  let  the  propeller  be 
at  rest  and  the  machine  be  making  a  straightaway  glide  to  earth, 
and  let  it  suddenly  pass  in  to.  a  lower  layer  of  air  moving  in  the  same 
horizontal  direction  as  the  machine  and  with  the  same  velocity. 
This,  of  course,  is  an  extreme  case,  but  it  is  by  no  means  an  impossible 
one.  Instantly  on  entering  the  lower  layer,  under  the  conditions 
just  described,  all  dynamical  support  must  cease,  and  with  it  all  power 
of  guidance.  A  fall,  for  at  least  a  considerable  distance,  is  absolutely 
inevitable,  and,  if  near  the  earth,  perhaps  a  disastrous  one.  To  all 
intents  and  purposes  a  hole  has  been  run  into. 

The  reason  for  the  fall  will  be  understood  when  it  is  recalled  that 
the  pressure  of  any  ordinary  wind  is  very  nearly  proportional  to  the 
square  of  its  velocity  with  respect  to  the  thing  against  which  it  is 
blowing.  Hence,  for  a  given  inclination  of  the  wings,  the  lift  on 
an  aeroplane  is  approximately  proportional  to  the  square  of  the 
velocity  of  the  machine  with  reference,  not  to  the  ground,  but  to  the 
air  in  which  it  happens  to  be  at  the  instant  under  consideration.  If, 
then  it  glides,  with  propellers  at  rest,  into  a  wind  stratum  that  is 
blowing  in  the  same  horizontal  direction  and  with  the  same  velocity 
it  is  in  exactly  the  condition  it  would  be  if  dropped  from  the  top  of  a 
monument  in  still  air.  It  inevitably  must  fall  unless  inherent 
stability  or  skill  of  the  pilot  bring  about  a  new  glide  after  additional 
velocity  had  been  acquired  as  the  result  of  a  considerable  drop. 

Of  course,  such  an  extreme  case  must  be  of  rare  occurrence,  but 
cases  less  extreme  are  met  with  frequently.  On  passing  into  a 
current  where  the  velocity  of  the  wind  is  more  nearly  that  of  the 
aeroplane,  and  in  the  same  direction,  part  of  the  supporting  force 
is  instantly  lost  and  a  corresponding  drop  or  dive  becomes  at  once 
inevitable.  Ordinarily,  however,  this  is  a  matter  of  small  conse- 
quence for  the  relative  speed  necessary  to  support  it  is  again  soon 
acquired,  especially  if  the  engine  is  in  full  operation.  Occasionally, 
though,  the  loss  in  support  may  be  large  and  occur  but  a  short 
distance  above  the  ground  and,  therefore,  be  correspondingly  dan- 
gerous. 

If  the  new  wind  layer  is  against  and  not  with  the  machine,  an 
increase  instead  of  a  decrease  in  the  sustaining  force  is  the  result, 
and  but  little  occurs  beyond  a  mere  change  in  the  horizontal  speed 
with  reference  to  the  ground  and  a  slowing  up  of  the  rate  of  descent. 

All  the  above  discussion  of  the  effect  of  wind  layers  on  aeroplanes 
is  on  the  assumption  that  they  flow  in  parallel  directions.  Ordi- 
narily, however,  they  flow  more  or  less  across  each  other  and  there- 
fore, as  a  rule,  the  aviator  on  passing  out  of  one  of  them  into  the 


MANUAL  OF   AEROGRAPH Y.  47 

other  has  to  contend  with  more  than  a  disconcertingly  abrupt  change 
in  the  supporting  force.  That  is  to  say,  on  crossing  the  interface 
between  wind  sheets,  in  addition  to  suffering  a  partial  loss  of  support, 
he  usually  has  to  contend  with  the  turmoil  of  a  choppy  aerial  sea, 
in  which  "bumps"  at  least  seem  to  abound  everywhere. 

Wind  strata  within  ordinary  flying  levels  are  most  frequent  dur- 
ing weather  changes,  especially  as  fine  weather  is  giving  way  to 
stormy.  This,  then,  is  a  time  to  be  on  one's  guard  against  the 
most  troublesome  of  all  ''holes  in  the  air,"  even  to  the  extent  of 
making  test  soundings  for  them  with  small  pilot  balloons.  It  is  also 
well  on  such  occasions  to  avoid  making  great  changes  in  altitude, 
since  wind  strata,  of  whatever  intensity,  remain  roughly  parallel  to 
the  surface  of  the  earth,  and  the  greater  the  change  in  altitude  the 
greater  the  risk  of  running  into  a  treacherous  ''hole/'  Also  to  avoid 
the  possibility  of  losing  support  when  too  low  to  dive,  and  for  other 
good  reasons,  landings  and  launchings  should  be  made,  if  practicable, 
squarely  in  the  face  of  the  surface  wind. 

Wind  billows. — It  was  stated  above  that  when  one  layer  of  air 
runs  over  another  of  different  density  billows  are  set  up  between 
them,  as  often  shown  by  windrow  clouds;  however,  the  warning 
clouds  are  comparatively  seldom  present,  and  therefore  even  the 
cautious  aviator  may,  with  no  evidence  of  danger  before  him,  take 
the  very  level  of  the  air  billows  themselves  and,  before  getting  safely 
above  or  below  them  encounter  one  or  more  sudden  changes  in  wind 
velocity  and  direction  due,  in  part,  to  the  eddy-like  or  roUing  motion 
within  the  waves,  with  chances  in  each  case  of  being  suddenly  de- 
prived of  a  large  portion  of  the  requisite  sustaining  force — of  en- 
countering a  "hole  in  the  air."  There  may  be  perfect  safety  in 
either  layer,  but,  unless  headed  just  right,  there  necessarily  is  some 
risk  in  going  from  one  to  the  other.  Hence,  flying  at  the  billow 
level,  since  it  would  necessitate  frequent  transitions  of  this  nature, 
should  be  avoided. 

When  the  billows  are  within  300  meters,  say,  or  less  of  the  earth 
(often  the  case  during  winter,  owing  to  the  prevalence  then  of  cold 
surface  air  with  warmer  air  above),  they  are  apt  to  be  very  turbulent, 
just  as  and  for  much  the  same  reason  that  waves  in  shallow  water 
are  turbulent. 

For  this  reason,  presumably,  winter  flying  sometimes  is  surprisingly 
rough,  the  air  is  full  of  " bumps"  and  "holes."  Fortunately,  how- 
ever, it  is  easy  to  determine  by  the  aid  of  a  suitable  station  barograph 
whether  or  not  billows  are  prevalent  in  the  low  atmosphere,  since 
they  produce  frequent  (5  to  12  per  hour,  roughh')  pressure  changes, 
usually  of  0.1  Kb.  to  0.4  Kb.  at  the  surface. 

Wind  gusts. — Near  the  surface  of  the  earth  the  wind  is  always  in  a 
turmoil,  owing  to  friction  and  to  obstacles  of  all  kinds  that  interfere 
with  the  free  flow  of  the  lower  layers  of  the  atmosphere,  and  thereby 


48  MANUAL  OF   AEROGRAPHY. 

allow  the  next  higher  layers  to  plunge  forward  in  irregular  fits, 
swirls,  and  gusts,  with  all  sorts  of  irregular  velocities  and  in  every 
which  direction.  Indeed,  the  actual  velocity  of  the  wind  near  the 
surface  of  the  earth  often  and  abruptly  varies  from  second  to  second 
by  more  than  the  full  average  value,  and  the  greater  the  average 
velocity,  the  greater,  in  approximately  the  same  ratio,  are  the  irregu- 
larities or  differences  in  the  successive  momentary  velocities. 

Clearly,  in  a  wind  of  this  kind,  if  at  all  violent,  the  support  to  an 
aeroplane  is  correspondingly  erratic  and  varies  between  such  wide 
limits  that  the  aviator  finds  himself  in  a  veritable  nest  of  "holes" 
out  of  whicn  it  is  difficult  to  rise,  at  least  with  a  slow  machine,  and 
sometimes  dangerous  to  try.  However,  as  the  turmoil  due  to  hori- 
zontal winds  rapidlv  decreases  with  increase  of  elevation,  and  as  the 
aviator's  safety  depends  upon  steady  conditions,  or  upon  the  velocity 
of  his  machine  with  reference  to  the  atmosphere  and  not  with  refer- 
ence to  the  ground,  it  is  obvious  that  the  windier  it  is  the  higher  in 
general  the  minimum  level  at  which  he  should  fly. 

Wind  eddies. — Just  as  eddies  and  whirls  exist  in  every  stream  of 
water,  from  tiny  rills  to  the  great  rivers,  and  even  the  ocean  currents, 
wherever  the  banks  are  such  as  greatly  to  change  the  direction  of 
flow  and  wherever  there  is  a  pocket  of  considerable  depth  and  extent 
on  either  side,  and  as  similar  eddies,  but  with  horizontal  instead  of 
vertical  axes,  occur  at  the  bottom  of  streams  where  they  flow  over 
ledges  that  produce  abrupt  changes  in  the  levels  of  the  beds,  so,  too, 
and  for  the  same  general  reasons,  horizontal  eddies  occur  in  the 
atmosphere  with  rotation  proportional,  roughly,  to  the  strength  of 
the  wind.  These  are  most  pronounced  on  the  lee  sides  of  cuts,  cliffs, 
and  steep  mountains,  but  occur  also,  to  a  less  extent,  on  the  wind- 
ward sides  of  such  places. 

The  air  at  the  top  and  botton  of  such  whirls  is  moving  in  diamet- 
rically opposite  directions — at  the  top  with  the  parent  or  prevailing 
wind,  at  the  bottom  against  it — and  since  they  are  close  to  the  earth 
they  may,  therefore,  as  explained  under  "wind  layers,"  be  the  source 
of  decided  danger  to  aviators.  There  may  be  some  danger  also  at  the 
forward  side  of  the  eddy  where  the  downward  motion  is  greatest. 

When  the  wind  is  blowing  strongly  landings  should  not  be  made, 
if  at  all  avoidable,  on  the  lee  sides  of  and  close  to  steep  mountains, 
hills,  bluffs,  or  even  large  buildings,  for  these  are  the  favorite  haunts, 
as  just  explained,  of  treacherous  vortices.  The  whirl  is  best  avoided 
by  landing  in  an  open  place  some  distance  from  bluffs  and  large 
obstructions,  or,  if  the  obstruction  is  a  hill,  on  the  top  of  the  hill 
itself.  If,  however,  a  landing  to  one  side  is  necessary  and  the  aviator 
has  choice  of  sides,  other  things  being  equal,  he  should  take  the 
windward  and  not  the  lee  side.  Finally,  if  a  landing  close  to  the  lee 


.MAN  I' A  I.    OF    AKKO<;KAI'JIY.  49 

side  is  compulsory  he  should,  if  possible,  head  along  the  hill,  and  not 
toward  or  from  it;  along  the  axis  of  the  eddy  and  not  across  it.  Such 
a  landing  would  be  safe  unless  made  in  the  down  draft,  since  it  would 
keep  tho  machine  in  winds  of  nearly  constant  (zero)  velocity  with 
reference  to  its  direction,  whatever  the  side  drift,  provided  the  hill 
was  of  uniform  height  and  slope  and  free  from  irregularities.  But  as 
hills  seldom  fulfill  these  conditions  lee-side  landings  of  all  kinds 
should  l)r  avoided  whenever  there  is  any  considerable  wind. 

Air  torrents. — Just  as  water  torrents  are  duo  to  drainage  down 
steep  slopes,  so,  too,  air  torrents  owe  their  origin  to  drainage  down 
steep  narrow  valleys.  Whenever  the  surface  of  the  earth  begins  to 
cool  through  radiation  or  otherwise  the  air  in  contact  with  it  becomes 
correspondingly  chilled  and,  because  of  its  increased  density,  flows 
away  to  the  lowest  level.  Hence,  when  the  weather  is  clear,  there  is 
certain  to  be  air  drainage  down  almost  any  steep  valley  during  the  late 
afternoon  and  most  of  the  night.  When  several  such  valleys  run  into 
a  common  one,  like  so  many  tributaries  to  a  river,  and  especially  when 
the  upper  reaches  contain  snow  and  the  whole  section  is  devoid  of 
forest,  the  aerial  river  is  likely  to  become  torrential  in  nature  along 
the  lower  reaches  of  the  drainage  channel. 

A  flying  machine  attempting  to  land  in  the  mouth  of  such  a  valley 
after  the  air  drainage  is  well  begun  is  in  danger  of  going  from  rela- 
tively quiet  air  into  an  atmosphere  that  is  moving  with  considerable 
velocity,  at  times  amounting  almost  to  a  gale.  If  one  must  land  at 
such  a  place,  he  should  head  up  the  valley  so  as  to  face  the  wind.  If 
he  heads  down  the  valley  and  therefore  runs  with  the  wind,  he  will, 
on  passing  into  the  swift  air,  lose  his  support,  or  much  of  it,  for 
reasons  already  explained,  and  correspondingly  drop. 

Air  breakers. — The  term  "air  breakers"  is  used  here  in  analogy 
with  water  breakers  as  a  general  name  for  the  rolling,  dashing,  and 
choppy  winds  that  accompany  thunderstorm  conditions.  They  often 
arc  of  such  violence,  up,  down,  and  sideways  in  any  and  every  direc- 
tion, that  an  aeroplane  in  their  grasp  is  likely  to  have  as  uncontrolled 
and  disastrous  a  landing  as  would  be  the  case  in  an  actual  "hole" 
of  the  worst  kind. 

Fortunately  air  breakers  usually  give  abundant  and  noisy  warnings, 
and  hence  the  cautious  aviator  need  seldom  be  and,  as  a  matter  of 
fact,  seldom  is  caught  in  so  dangerous  a  situation.  However,  more 
than  one  disaster  is  attributable  to  just  such  winds  as  these — to  air 
breakers. 

Classification. — The  above  10  types  of  atmospheric  conditions 
may  conveniently  be  divided  into  two  groups  with  respect  to  the 
method  by  which  they  force  an  aeroplane  to  drop. 

1.  The  vertical  group. — All  those  conditions  of  the  atmosphere, 
such  as  air  fountains,  cataracts,  cloud  currents,  cascades,  breakers,  and 
50821—18 4 


50  MANUAL  OF  AEROGRAPHY. 

eddies  (forward  side),  that,  in  spite  of  full  speed  ahead  with  reference 
to  the  air,  make  it  difficult  or  impossible  for  the  aviator  to  maintain 
his  level,  belong  to  a  common  class  and  depend  for  their  effect  upon 
a  vertical  component,  up  or  down,  in  the  motion  of  the  atmosphere 
itself.  Whenever  the  aviator,  without  change  of  the  angle  of  attack 
and  with  a  full  wind  in  his  face  finds  his  machine  rapidly  sinking,  he 
may  be  sure  that  he  has  run  into  some  sort  of  a  down  current.  Ordi- 
narily, however,  assuming  that  he  is  not  in  the  grasp  of  storm  breakers, 
this  condition,  bad  as  it  may  seem,  is  of  but  little  danger.  The  wind 
can  not  blow  into  the  ground  and  therefore  any  down  current  however 
vigorous  must  somewhere  become  a  horizontal  current,  in  which  the 
aviator  may  fly  away  or  land  as  he  chooses. 

2.  The  horizontal  group. — This  group  includes  all  those  at- 
mospheric conditions — wind  layers,  billows,  eddies  (central  portion), 
torrents,  and  the  like — that,  in  spite  of  full  speed  ahead  with  reference 
to  the  ground,  abruptly  deprive  an  aeroplane  of  a  portion  at  least  of 
its  dynamical  support.  When  this  loss  of  support,  due  to  a  running 
of  the  wind  more  or  less  with  the  machine,  is  small  and  the  elevation 
sufficient,  there  is  but  little  danger;  but,  on  the  other  hand,  when  the 
loss  is  relatively  large,  especially  if  near  the  ground,  the  chance  of  a 
fall  is  correspondingly  great. 

W.  J.  H. 


CHAPTER  V. 


STORMS  AND  STORM  TRACKS. 


51 


CHAPTER  V. 


STORMS  AND  STORM  TRACKS. 

Cyclones  and  Anticyclones. — On  the  daily  weather  maps  of  the 
Weather  Bureau  are  regions  marked  "high"  and  "low"  and  bounded 
by  a  number  of  circular  or  oval  lines.  These  lines,  called  isobars, 
connect  places  of  equal  barometric  pressure.  From  these  charts  it 
is  evident  that  there  is  a  distinct  relation  between  the  distribution 
of  pressure,  the  direction  and  force  of  the  wind,  the  temperature,  and 
weather  generally.  By  glancing  at  a  number  of  maps  it  will  be 
noticed  that  there  is  almost  always  an  area  of  low  barometric  pressure, 
called  a  "cyclone,"  having  an  oval  form  and  generally  moving  in  an 
easterly  or  northeasterly  direction;  or  a  region  of  high  barometric 
pressure,  called  an  "anticyclone,"  also  oval  in  form  and  moving  more 
slowly  in  the  same  direction.  The  winds  in  both  cases  tend  to  blow 
nearly  parallel  with  the  isobars,  having  the  region  of  lowest  pressure 
on  the  left  hand.  This  relation  is  expressed  in  Buys-Ballot's  law  (for 
the  Xorthern  Hemisphere;  Southern,  opposite),  "Stand  with  your 
back  to  the  wind,  and  the  barometer  will  be  lower  on  your  left  than 
on  your  right." 

Cyclone. — The  weather  in  a  cyclone  is  described  in  the  following 
outline.  In  the  extreme  front  of  a  cyclone  there  is  a  blue  sky;  then, 
with  the  barometer  falling,  cirro-stratus  clouds  appear  (with  probably 
a  halo),  which  gradually  becomes  lower  and  denser  and  forms  an 
overcast  sky.  The  temperature  rises  and  the  air  feels  muggy  and 
close.  As  the  center  of  the  area  approaches,  rain  falls  until  the 
barometer  begins  to  rise.  The  passage  of  the  trough  is  often  accom- 
panied by  a  heavy  shower  or  squall,  known  as  the  clearing  shower. 
The  air  then  cools,  loses  its  mugginess,  and  soon  patches  of  blue  sky 
appear.  The  shift  of  wind  is  different  in  the  right-hand  portion  of 
the  "low"  area  than  it  is  in  the  left-hand  portion.  In  the  right-hand 
portion  the  flow  is  S.  to  SE.,  and  as  the  area  moves  along  there  is  a 
gradual  change  to  SW.  and  W.,  with  increase  in  velocity.  In  the 
left-hand  side  the  flow  is  S.  to  SE.,  then  on  to  E.,  NE.,  N.,  and,  finally, 
X\V.,  when  there  is  a  decrease  and  the  weather  becomes  fair.  The 
change  of  direction,  and  consequently  the  accompanying  weather, 
depends  upon  the  area  passing  to  the  north  or  south  of  us. 

Anticyclone. — In  an  anticyclone  the  weather  is  almost  the  oppo- 
site to  that  in  a  cyclone;  the  weather  of  a  cyclone  is  unsettled,  while 

53 


54 


MANUAL   OF   AEROGRAPHY. 


that  in  a  cyclone  is  settled  and  fair.  In  an  anticyclone  the  sky  is 
generally  clear  and  the  air  calm,  consequently  the  temperature  rela- 
tively high  during  the  day  and  low  at  night.  In  the  winter  there  is 
often  frost,  frequently  accompanied  by  fog.  The  wind  directions  are 
clockwise  and  slightly  outwards;  and,  as  the  movement  of  a  "high" 
is  sluggish,  often  blows  from  the  same  quarter  for  comparatively  long 
periods.  (See  table.) 

There  is  sometimes  a  ridge  of  high  pressure  between  two  cyclones. 
When  this  occurs  there  is  usually  a  short  period  of  fine  weather. 
The  sun  is  hot  during  the  day  and  there  is  great  radiation  at  night. 

Comparative  table. 


Cyclone. 

Anticyclone. 

Diameter 

One  hundred  to  several  hundred  miles. 

Varies  948  to  little  less  than  1016  kbs., 
approximately. 
Spirally  inflowing:  counter  clockwise.  . 
Outside,  moderate:  center,  strong   to 
high;  center,  light. 
Fast  

Several  hundred  to  several  thousand, 
miles. 
Varies    1016    to    1033    kbs.,   approxi- 
mately. 
Spirally  outflowing;  clockwise. 
Moderate  with  frequent  calms. 

Slow:  sluggish. 
Nearly  circular  or  oval. 
N-S:  NE-SW;  NW-SE. 
Front  ,  cool  :  rear  ,  warm  . 
Front.  low  humidity;  rear,  high  humid- 
ity. 

Barometric  pressure  
Winds...     . 

Wind  velocity  

Movement  .  .  . 

Form 

Nearly  circular  or  oval 

Axis  direction  .  .  . 

N-S;  NE-SW:  NW-SE  .  . 

Heat  distribution  
Moisture.. 

Front  ,  warm:  rear  .  cool  
Front  ,  hi  eh  humidity;  rear,  low  humid- 
ity. 

Secondary  storms  are  local  storms  generally  forming  within  the 
southern  quadrants  (in  the  northern  hemisphere,  opposite  in  south- 
ern) of  a  larger  cyclonic  disturbance,  and  usually  follow  in  the  same 
general  direction  as  the  larger  storm.  Tornadoes,  waterspouts,  and 
thunderstorms  are  of  this  type. 

THUNDERSTORMS . 

The  frequency  of  thunderstorms  depends  mainly  upon  the  exist- 
ence of  conditions  necessary  to  produce  much  local  convection.  The 
warmer  the  atmosphere  the  greater  the  convection  and  vapor  content. 
Consequently,  the  frequency  is  greatest  on  the  land  during  the  heat 
of  day  (afternoons),  the  hottest  months  of  the  year  (June,  July,  and 
August),  and  over  the  ocean  at  its  hottest  period.  (The  high  specific 
heat  of  water  results  in  its  losing  its  heat  considerably  slower  than 
the  land  and  we  have  the  best  conditions  for  vertical  convection  in 
the  early  morning  hours.) 

Thunderstorms  are  relatively  small  atmospheric  whirls.  During 
their  occurrence  the  wind  at  the  surface  is  light  or  calm.  There  is 
considerable  motion  in  the  clouds  above,  however,  which  sometimes 
become  violent;  and  as  the  clouds  of  this  type  are  comparatively 
low,  we  surmise  that  the  cyclonic  motion  is  confined  to  the  surface 
layer.  Thunderstorm  whirls  may  vary  from  approximately  1  to  10 


MANUAL   OF   AEROGRAPm  .  55 

kilometers  in  diameter.  When  the  storm  is  of  low  altitude  strong 
winds  roach  the  surface.  Often  there  are  several  storms  over  the 
same  locality  which  follow  one  another  along  the  same  track.  They 
follow  the  "  path  of  least  resistance,"  and  consequently  tend  to  confine 
themselves  to  low  ground,  valleys,  etc.  The  areas  affected  by  single 
storm-  are  nearly  always  narrow  and  diversified,  as  traced  by  amount 
of  rainfall,  and  it  is  not  unusual  to  find  storms  taking  parallel  paths 
with  nearly  dry  areas  between.  Thunderstorms  average  about  30 
kilometers  an  hour  in  movement  in  low  barometric  areas,  and  at  a 
higher  rate  in  squally  conditions  accompanying  supplementary  de- 
pressions, or  "secondaries." 

There  is  also,  another  type  of  thunderstorm  to  which  Sir  Napier 
Shaw  gives  the  name  of  "line  thunderstorms."  In  this  type  the 
area  of  simultaneous  thunder  disturbance  extends  along  a  ''line" 
which  may  be  160  kilometers  in  length.  This  line  moves  parallel  to 
itself,  and  usually  takes  a  direction  from  northwest  to  southeast, 
with  a  velocity  of  about  80  kilometers  per  hour. 

Tornado. — A  type  of  secondary  storm  peculiar  to  the  United 
States  east  of  the  Rocky  Mountains,  usually  occurring  in  connection 
with  a  thunderstorm  and  attaining  destructive  velocity.  The 
counter-clockwise  winds  accompanying  a  tornado  range  from  45  to 
225  meters  per  second.  The  diameter  averages  about  300  meters; 
the  length  of  its  path  is  from  100  to  500  kilometers;  and  its  move- 
ment about  11  meters  per  second. 

The  conditions  most  favorable  to  the  formation  of  tornadoes  are: 
Counter  wind  currents  of  widely  different  temperatures  causing 
strong  vertical  convection.  These  conditions  occur  most  frequently 
in  spring  and  summer  months  and  hi  the  warm  sectors  bounding  the 
Gulf  of  Mexico.  During  the  formation  there  is  great  turbulence  in 
the  clouds.  This  turbulence  results  in  the  formation  of  a  funnel- 
shaped  cloud  beneath  the  center  of  the  clouds,  which  gradually 
drops  toward  the  surface  in  a  spirally  moving  column,  reaching  the 
ground  or  hovering  just  above  it.  When  it  does  reach  the  surface 
it  moves  in  a  northeasterly  direction,  spreading  destruction  in  its 
immediate  path. 

Tornadoes  are  the  result  of  mechanical  action.  As  stated  before, 
we  have  winds,  blowing  in  approximately  opposite  directions,  and 
of  widely  different  temperatures.  There  is  also  an  "intermingling" 
and  "overrunning"  of  counter  currents,  which  results  in  an  indraft 
feeding  the  upward  current  immediately  preceding  a  thunderstorm, 
and  produces  a  violent  whirl.  This  whirl  tends  to  draw  the  air 
under  it  into  itself,  and  the  continuance  of  this  action  results  in  a 
formation  of  a  spirally  rising  column.  The  force  of  a  tornado  may 
be  broken  or  its  course  altered  by  hills  or  mountains,  but  buildings, 


56  MANUAL  OF   AEROGRAPH Y. 

trees,  etc.,  standing  in  its  immediate  path  are  destroyed  without  any 
effect  upon  its  course. 

Waterspouts. — A  type  of  secondary  storm  which  results  in  a 
geyser-like  spiral  column  of  water,  reaching  from  the  sea  to  the 
thundercloud  wherein  they  are  formed.  They  are  caused  by  con- 
ditions similar  to  those  preceding  formation  of  tornadoes  on  land. 
They  are  of  comparatively  short  duration,  seldom  holding  their 
form  for  more  than  an  hour;  and  on  account  of  their  limited  area  of 
action,  can  not  be  classed  as  destructive. 

STOKM   TRACKS. 

Storm  tracks  are  made  by  the  connection  of  continuous  areas  of 
lowest  barometric  pressure.  Storm  tracks  over  the  Atlantic  Ocean 
are  shown  on  the  monthly  charts  issued  by  the  Hydrographic  Office, 
United  States  Navy,  Washington,  D.  C. ;  over  the  United  States,  in 
the  publications  of  the  United  States  Weather  Bureau,  Washington, 
D.  C. 

J.  W.  A.  B. 


CHAPTER  VI. 


PRESSURE. 


57 


CHAPTER  VI. 


PRESSURE. 

Historical  data. — The  nature  of  the  atmosphere  and  of  atmos- 
pheric phenomena  have  been  matters  of  conjecture  and  study  since 
ancient  times.  In  spite  of  the  importance  of  the  subject  little  progress 
was  made  in  the  study  until  the  seventeenth  century,  when  Toiricelli 
developed  the  first  barometer  and  demonstrated  that  atmosphere 
had  weight  and  exerted  a  certain  hydrostatic  pressure.  Pascal  showed 
that  the  atmospheric  pressure  decreased  with  elevation  and  could  be 
detected  even  at  such  small  differences  in  elevation  as  50  meters. 
At  the  same  time  von  Gueiicke  demonstrated  that  the  pressure  rose 
and  fell  with  differences  in  atmospheric  conditions  and  that  weather 
changes  as  well  as  differences  in  elevation  could  be  detected  by  the 
use  of  the  barometer. 

While  a  study  of  the  upper  atmosphere  progressed  gradually  from 
the  eighteenth  century  on,  it  was  not  until  the  latter  part  of  the 
nineteenth  century  that  the  exploration  and  systematic  measurement 
of  the  various  upper  layers  of  atmosphere  were  begun. 

Following  the  pioneer  work  of  Rotch  at  Blue  Hill  Observatory  and 
Teisserenc  de  Bort  at  Trappes,  systematic  investigations  of  the 
upper  air  were  carried  on  at  many  of  the  leading  meteorological 
stations  throughout  the  world.  We  have  to-day  the  results  of  these 
investigations,  but  as  yet  the  upper  air  study  is  in  its  infancy. 

Height  of  atmosphere. — Before  proceeding  with  the  question  of 
pressure  it  is  well  to  consider  and  keep  in  mind  that  the  atmosphere 
has  considerable  thickness  or  height.  Astronomers  have  estimated 
the  height,  based  on  observations  of  the  paths  of  shooting  stars  or 
meteors,  to  be  from  150  to  200  kilometers.  Other  estimates  have 
been  made  from  measurements  of  such  atmospheric  phenomena  as 
the  auroral  arc  and  the  twilight  arch.  In  the  case  of  the  latter  method 
the  height  is  estimated  to  be  79  kilometers,  or  50  miles,  an  estimate 
that  is  probably  25  per  cent  too  much  on  account  of  the  refraction 
of  rays  as  they  pass  through  the  layers  of  air  of  different  densities. 
Never  having  been  able  to  observe  the  free  surface  of  a  gas  we  can 
not  tell  much  about  the  limits  of  the  atmosphere,  but  it  is  safe  to  say 
that  it  extends  to  much  greater  elevations  than  30  kilometers. 

Distribution  of  gases  in  the  atmosphere. — While  of  minor 
importance  in  our  study  of  the  air  as  a  medium  of  navigation,  it  may 

59 


60 


MANUAL  OF   AEROGRAPHY. 


be  well  to  have  some  idea  of  the  gases  in  the  atmosphere.  While  the 
early  meteorologists  assumed  that  the  atmosphere  was  homogeneous 
and  the  gases  were  uniformly  distributed  according,  to  temperature, 
pressure  and  density,  later  investigations  proved  that  such  was  not 
the  case,  because  the  atmosphere  is  characterized  by  well  marked 
circulations  and  continuous  departure  from  any  state  of  rest. 

According  to  Hann,1  the  chief  independent  gases  that  are  blended 
into  a  dry  atmosphere  at  the  surface  of  the  earth,  and  their  re- 
spective volume  percentages,  are  as  follows: 

Per  cent. 

Nitrogen 78.  03 

Oxygen 20.  99 

Argon.  % 94 

Carbon  dioxide 03 

Hydrogen ^ 01 

Neon 0013 

Helium ..  0004 

Besides  these  gases  and  a  few  of  minor  importance  such  as  krypton 
and  xenon,  there  are  many  substances,  chief  among  which  is  water 
vapor,  that  are  found  in  vaiying  amounts  in  the  atmosphere.  The 
percentage  of  water  vapor  present  in  the  air  varies  according  to  the 
temperature  as  warmer  air  contains  a  greater  weight  of  water  vapor 
at  saturation  than  air  at  a  lower  temperature.  Because  of  the  rela- 
tion of  water  vapor  to  temperature  the  percentage  volume  decreases 
from  the  equator  to  the  poles,  while  the  percentage  volume  of  the 
other  gases  in  the  surface  atmosphere  is  substantially  the  same  at 
all  parts  of  the  earth.  Although  the  amount  of  water  vapor  even  at 
the  surface  is  only  1.2  per  cent  of  the  total  volume  of  gas,  it  is  ex- 
tremely important  in  our  consideration  of  pressure  and  later  of 
temperature. 

TABLE  A. — Percentage  distribution  of  gases  in  the  atmosphere. 


Height 
in  kilo- 
meters. 

Argon. 

Nitro- 
gen. 

Water 
vapor. 

Oxygen. 

Carbon 
dioxide. 

Hydro- 
gen. 

Helium. 

Total 
pressure  in 
kilobars. 

140 

0  01 

99  15 

0  84 

0  0053 

130  .. 

.04 

99.00 

.96 

.0060 

120 

19 

98  74 

1  07 

0069 

110  

.67 

0.02 

0.02 

98.10 

1.19 

.0079 

100 

2.95 

05 

11 

95  58 

1  31 

0089 

90  

9.78 

.10 

.49 

88.28 

1.35 

.0108 

80 

32.18 

17 

1  85 

64  70 

1  10 

0164 

70. 

0.03 

61.83 

.20 

4.72 

32.61 

.61 

.0365 

60. 

.03 

81.22 

.15 

"7.69 

10.68 

.23 

.1246 

50. 

.12 

86.78 

.10 

10.17 

2.76 

.07 

.4373 

40. 

.22 

86.42 

.06 

12.61 

.67 

.02 

2.  4530 

30. 
20 

.35 
.59 

84.26 
81.24 

.03 
.02 

15.18 
18.10 

0.01 
01 

.16 
.04 

.01 

11.5055 
54  6478 

15.. 
11. 

.77 
.94 

79.52 
78.02 

.01 
.01 

19.66 
20  99 

.02 
.03 

.02 
01 

119.  5347 
223.  9776 

5... 
0.. 

.94 
.93 

77.89 
77.08 

.18 
1.20 

20.95 
20.75 

.03 
.03 

.01 
.01 

539.  9460 
1  013.2320 

From  Physics  of  the  Air.    (Humphreys. ) 

i  Lehrbuch  der  Meterology,  3d  edition. 


MANUAL   OF   AEROGEAPHY.  61 

The  amount  of  water  vapor  (or  absolute  humidity)  rapidly  de- 
creases under  the  influence  of  the  lower  temperatures  with  increase 
of  elevation  to  a  negligible  amount  at  or  below  the  level  of  10  kilo- 
meters. By  assuming,  first,  that  the  temperature  decreases  uni- 
formly at  the  rate  of  22  kk.  (6  k.)  per  kilometer  from  1040  kk. 
284  k.)  at  the  surface,  to  800  kk.  (218  k.)  at  11  kilometers,  and 
above  this  elevation,  remains  constant  at  800  kk.;  second,  that 
vertical  convection  occurring  in  the  region  of  constant  temperature 
changes  from  the  surface  up  to  the  region  of  constant  temperature, 
which  keeps  the  relative  per  cent  of  these  several  gases,  with  the  excep- 
tion of  water  vapor,  constant;  and,  third,  that  above  this  elevation 
in  the  region  of  constant  temperatures  and  little  or  no  vertical  con- 
vection, the  several  gases  are  distributed  according  to  their  respec- 
tive molecular  weights.  Humphreys,  in  Physics  of  the  Air  *  has 
computed  by  the  aid  of  several  barometric  hypsometric  formulas 
the  approximate  composition  and  barometric  pressure  of  the  atmos- 
phere at  various  levels,  as  given  in  Table  A.  In  studying  this  table, 
it  must  be  kept  in  mind  that  the  figures  are  supported  by  direct 
experimental  observations  only  from  the  surface  of  the  earth  up  to 
a  level  of  about  30  kilometers,  and  that  while  above'  this  level  the 
values  are  based  on  sound  logic,  they  are  nevertheless  less  certain 
with  increase  in  elevation.  While  the  percentage  of  water  vapor 
reaches  a  certain  maximum  at  an  elevation  of  70  to  80  kilometers, 
the  amount  decreases  with  elevation,  but  less  rapidly  than  do  the 
heavier  constituents,  and  more  rapidly  than  the  two  lighter  ones, 
hydrogen  and  helium. 

Atmospheric  circulation. — So  far  we  have  considered  the  atmos- 
phere as  having  considerable  thickness,  and  consisting  of  a  mixture 
of  gases  that  increase  in  density  as  we  approach  the  earth.  These 
gases  have  weight,  and  exert  a  downward  pressure  due  to  the  pull  of 
gravity,  which  decreases  with  elevation.  If  there  were  no  tem- 
perature differences  or  winds  to  cause  a  difference  in  density,  and 
if  the  pull  of  gravity  were  the  same  at  all  points  on  and  above  the 
earth's  surface,  the  pressure  would  be  the  same  at  all  points  at  the 
same  elevation,  and  would  decrease  at  a  constant  rate  as  we  go  to 
higher  elevations.  This  is  not  the  case,  however,  and  the  pressure 
varies  at  different  points  on  the  same  level,  is  constantly  changing 
at  any  point,  and  does  not  decrease  at  the  same  rate  for  any  unit 
rise  in  elevation. 

The  existence  of  certain  great  air  streams  around  the  globe,  such 
as  the  trade  winds  and  monsoons,  have  been  known  for  a  considerable 
time,  and  many  explanations  of  their  causes  as  well  as  the  causes  of  air 
movements  in  general,  have  been  given.  The  precise  explanations 
of  these  theories  of  air  movements  are  problems  in  dynamics,  so  it  is 

i  Journal  of  The  Franklin  Institute,  Sept.,  1917. 


62  MANUAL  OF   AEROGRAPH Y. 

sufficient  to  state  here  that  there  are  three  causes  that  help  in  the 
establishment  of  the  major  circulations  of  the  globe,  and,  less  directly, 
the  minor  circulations.  These  are  (1)  the  unequal  heating  of  the  equa- 
torial and  polar  regions  (with  gravity  the  prime  factor  in  causing  air 
motion);  (2)  the  deflective  forces  due  to  the  earth's  rotation;  and 
(3)  the  unequal  absorption  of  heat  by  land  and  water  surfaces,  which 
determine  largely  the  location  of  the  hyperbars  and  infrabars,  or 
"centers  of  action."  1 

The  velocity  of  the  earth's  motion  is  greatest  at  the  Equator  and 
gradually  diminishes  toward  the  poles ;  so  that  a  current  of  air  flow- 
ing from  a  lower  to  a  higher  latitude  is  deflected  to  the  eastward; 
conversely,  a  current  of  air  flowing  from  a  higher  to  a  lower  latitude 
is  deflected  to  the  westward,  since  the  velocity  of  the  earth's  motion 
is  not  so  great  in  the  higher  as  it  is  in  the  lower  latitude.  Thus  in  the 
Northern  Hemisphere  a  current  of  air  flowing  northward  is  deflected 
to  the  right  and  becomes  a  southwesterly  wind,  and  a  current  flowing 
south  is  deflected  to  the  right  and  becomes  a  northeasterly  wind.  In 
the  Southern  Hemisphere  the  converse  is  true,  northerly  winds  and 
southerly  winds  are  both  deflected  to  the  left.  From  this  we  see  that 
when  air  currents  flow  toward  areas  of  low  pressure  from  high-pressure 
areas  they  are  deflected  to  the  right  in  the  Northern  Hemisphere  and 
to  the  left  in  the  Southern  Hemisphere  by  the  earth's  rotation,  and 
instead  of  flowing  directly  toward  the  center  of  depression,  will  acquire 
a  motion  around  it,  but  inclined  inward  toward  the  center.  For  this 
reason  the  wind  circulation  about  an  area  of  low  pressure  in  the 
Northern  Hemisphere  will  have  a  movement  counterclockwise,  while 
in  the  Southern  Hemisphere,  the  movement  will  be  clockwise.  The 
movement  around  areas  of  high  pressure  will  be  outward  and  con- 
verse in  direction.2  Briefly  stated,  moving  air  is  deflected  to  the 
right  in  the  Northern  Hemisphere,  and  to  the  left  in  the  Southern. 
Components  of  gravity  cause  the  deflection.  With  east  and  west 
motions  the  direction  but  not  the  velocity  is  affected.  Air  moving 
toward  the  pole  has  its  eastward  velocity  increased,  and  air  moving 
toward  the  Equator  has  its  velocity  diminished. 

Distribution  of  pressure. — The  distribution  of  pressure  over  the 
continents  and  oceans  determines  in  large  measure  the  path  and 
frequency  of  smaller  circulations.  We  find  by  a  study  of  the  pres- 
sure distribution  over  the  globe  that  there  are  certain  areas  of  high 
and  low  pressure  to  which  can  be  traced  periods  of  abnormal  weather. 

In  the  Pacific  we  find  two  areas  where  the  pressure  is  in  excess, 
sovcalled  hyperbars,  one  west  of  California  and  extending  southwest, 
and  the  other  over  the  Southern  Pacific  west  of  Chile.  Over  the 
Atlantic  there  are  two  hyperbars,  one  in  the  region  of  the  thirty-fifth 

1  Principles  of  Aerography.    (McAdie.) 

2  Seaman's  Handbook.    (W.  N.  Shaw.) 


MANUAL  OP  AEKOGRAPHY.  63 

north  parallel,  with  a  small  Bermuda  extension,  and  one  west  of 
southern  Africa.  Another  hyperbar  is  located  over  the  Indian 
Ocean,  and  one  in  the  vicinity  of  Australia.  The  land  hyperbars 
are  over  western  North  America,  southwestern  Europe,  and  central 
Asia.  There  are  certain  reversals  of  these  areas  between  summer 
and  winter,  as,  for  example,  the  North  American  hyperbar  becomes 
an  infrabar  in  summer,  and  the  Australian  hyperbar  of  July  (the 
winter  season)  becomes  an  infrabar  in  January  (the  summer  season). 
The  more  prominent  infrabars,  or  areas  of  diminished  pressure  at  the 
surface,  are  the  Aleutian  of  the  North  Pacific,  and  the  Icelandic  of 
the  North  Atlantic.  Thus  in  a  general  way  we  may  place  the  winter 
hyperbars  in  the  Northern  Hemisphere  between  latitudes  20°  and  40°, 
except  over  land  areas  where  they  extend  farther  north.  In  the 
Southern  Hemisphere  the  hyperbars  are  more  evenly  aligned,  and 
we  find  them  like  peaks  in  the  belt  of  prevailing  high  pressure  between 
latitudes  20°  and  40°.  The  distribution  of  these  ''centers  of 
action"  has  been  found  to  be  directly  related  to  the  character  of  the 
season.  For  example,  on  the  Pacific  slope  typical  wet  winters  occur 
when  the  North  Pacific  infrabar  overlies  the  continent  west  of  the  line 
drawn  from  Calgary  to  San  Francisco,  and  typical  dry  winters  occur 
when  the  continental  hyperbar  extends  westward  to  the  coast,  and 
the  Aleutian  infrabar  moves  to  the  northwest.  In  the  summer, 
with  the  Aleutian  infrabar  practically  disappearing,  the  continental 
hyperbar  moving  eastward,  and  the  oceanic  hyperbar  moving  north- 
ward, we  have  practically  a  rainless  period  over  this  same  area. 

The  strength  and  location  of  these  large  "centers  of  action"  de- 
termine the  frequency  and  path  of  the  individual  disturbances.  For 
example,  individual  "lows"  move  rapidly  southward,  when  the  con- 
tinental "high"  overlies  Idaho,  western  Washington,  and  eastern 
Oregon.  For  the  Atlantic  coast,  mild  winters  are  usually  associated 
with  the  presence  of  the  Bermuda  hyperbar,  while  low  temperatures 
result  from  its  continued  absence.1 

Ward,  in  his  discussion  of  cyclonic  and  anticyclonic  control  of  the 
weather  in  the  United  States,2  shows  that  the  spring  and  autumn 
transition  periods  are  marked  by  a  struggle  between  cyclonic  and 
solar  controls,  and  hence  by  striking  convectional  phenomena.  As 
summer  passes  the  sun's  rays  become  more  and  more  oblique,  and 
the  control  of  the  weather  passes  gradually  but  irregularly  from  the 
sun  back  again  to  the  cyclones,  or  "centers  of  action." 

Pressure  in  the  North  Atlantic  area. — As  we  are  especially 
interested  in  the  North  Atlantic  weather  forces,  since  they  affect  the 
British  Isles  and  the  coast  of  France  and  Belgium,  a  resume  of  the 
monthly  distribution  of  pressure  and  winds  over  this  particular  part 

i  Principles  of  Aerography .    ( Me  Adie. ) 

»  Annals  of  the  Association  of  the  American  Geog.,  vol.  4. 


64  MANUAL  OF   AEEOGEAPHY. 

of  the  globe  will  be  of  interest.  Over  the  North  Atlantic  area  the 
transition  from  winter  to  summer  shows  very  little  change  in  the 
general  distribution  of  pressure,  but  a  strengthening  of  the  pressure 
in  the  permanent  "high"  regions  of  the  thirty-fifth  parallel  and  a 
weakening  of  the  slope  to  the  Iceland  and  Greenland  "low."  The 
area  that  includes  the  British  Isles  and  the  coast  of  Franco  and 
Belgium  shows  merely  an  equalizing  of  pressure  from  south  to  north 
and  a  gradual  decline  of  intensity  of  the  conditions  rather  than  a 
change  of  general  type. 

In  the  winter  a  great  high-pressure  area  extends  from  Mexico 
across  the  Atlantic  to  southern  Europe  and  northern  Africa.  Above 
this  is  a  region  of  low-pressure  areas,  with  interruptions  due  to  varia- 
tions of  pressure  of  a  greater  or  less  degree. 

In  the  summer  the  high-pressure  area  of  the  Atlantic  just  north  of 
the  Tropic  of  Cancer  becomes  more  pronounced  and  more  isolated. 
The  intensity  of  the  low-pressure  region  farther  north  is  very  much 
reduced  and  there  is  a  tendency  for  the  "lows"  to  establish  themselves 
over  the  continental  areas  as  the  mean  temperature  of  the  day  in- 
creases. The  area,  including  the  British  Isles  and  the  coasts  of  France 
and  Belgium,  is  influenced  at  different  times  by  an  eastern  extension 
of  the  permanent  Atlantic  "high;"  by  the  continental  winter  "high;" 
by  an  extension  southeastward  of  the  "high"  in  the  vicinity  of 
Greenland;  and  by  the  "high"  which  usually  appears  over  Scandi- 
navia. These  influences  are  to  some  extent  seasonal,  but  may  be 
felt  at  any  time  during  the  year. 

With  weather  haying  a  general  west  to  east  movement,  the  British 
Isles,  located  as  they  are  at  the  western  margin  of  the  high-pressure 
area,  act  as  an  outpost  for  weather  forecasting  on  the  French  and 
Belgian  coast.1 

Cyclones  and  anticyclones. — It  has  been  assumed  that  low-pres- 
sure areas  are  caused  by  the  indraft  and  uplift  of  warm  moist  air, 
and  conversely  that  high-pressure  areas  are  due  to  descending  cold 
dry  air,  but  this  assumption  does  not  agree  with  observations;  and 
recent  study  of  the  structure  of  cyclones  and  anticyclones  shows  an 
entirely  different  origin  and  circulation.  Air  flow  is  the  result  of 
dynamic  rather  than  static  conditions,  and  the  surface  temperatures 
are  controlled  by  winds  rather  than  the  reverse.2 

Various  theories  of  the  cause  of  cyclonic  circulation  have  been 
advanced,  most  important  of  which  are  those  of  Harm  and  Bigelow. 
Hann  denied  the  existence  of  a  central,  warm,  uprising  current,  and 
held  that  the  actual  motion  of  the  atmosphere  is  not  the  direct  result 
of  surface  temperature,  but,  on  the  contrary,  temperature  may  be  the 
result  of  circulation.  He,  however,  used  as  the  prime  mover  the 
difference  of  temperature  between  the  poles  and  the  tropics  to  estab- 

i  Forecasting  the  Weather.    (Shaw.)  2  Winds  of  Boston.    (McAdie.) 


MANUAL   OF   AEROGRAPH Y.  65 

lish  the  circulation.  According  to  his  theory,  cyclones  and  anti- 
cyclones are  caused  by  the  irregular  flow  of  the  general  winds.  Air 
currents  moving  northward  in  consequence  of  temperature  gradients 
are  resolved  into  vortices  in  the  higher  latitudes,  and  the  movement 
of  these  vortices  is  determined  by  the  prevailing  direction  of  the  larger 
air  streams. 

Bigelow  in  his  later  theory  recognizes  the  asymmetry  of  cyclonic 
How  and  finds  its  origin  in  warm  and  cold  currents  arranged  in  ridges 
or  streams  of  different  densities  driven  into  local  circulation  by 
gravity.  This  does  away  with  warm  centered  cyclones  and  cold 
centered  anticyclones,  which  do  not  exist  except  in  the  hurricane  type 
of  tropical  storms.  The  cold  and  warm  currents  are  likewise  ac- 
counted for.  In  the  United  States  the  warm  currents  are  thrown  off 
by  the  Atlantic  hyperbar,  or  region  of  permanent  high  pressure.  Other 
warm  currents  are  from  the  Gulf  of  Mexico.  The  warm  currents  in 
southeastern  Asia  can  be  traced  to  the  Pacific  hyperbar.  The  con- 
tinents and  oceans  react  upon  the  general  circulation  and  greatly  dis- 
turb and  distort  its  free  operation,  so  that  finally  southern  currents 
prevail  in  certain  regions  and  northerly  currents  in  others.1 

It  is  evident  cyclonic  and  anticyclonic  structure  is  very  difficult 
to  explain.  Where  lower  air  strata  are  warm,  the  upper  strata  are 
cold.  The  idea  that  pressure  gradients  are  caused  by  temperature, 
and  that  they  in  turn  cause  the  winds,  is  superseded  by  the  idea  that 
winds  control  the  pressure  distribution  and  that  the  pressure  de- 
termines the  temperature  at  different  levels.  It  is  known  that  in  a 
cyclone,  when  we  go  some  distance  above  ground,  the  air  is  colder 
even  in  summer  than  in  an  anticyclone  in  winter.  Present  theories  do 
not  coincide  with  known  facts.  They  do  not  account  for  the  preva- 
lence of  the  westerly  winds  in  the  general  circulation,  and  the  upper 
air  temperatures  do  not  agree  with  those  required,  but  are  dependent 
upon  pressure  distribution.  Dines  and  others  have  shown  that  the 
temperature  of  the  upper  air  can  be  determined  more  accurately 
from  the  pressure  values  at  a  certain  level,  say  9  kilometers,  than 
from  any  known  seasonal  surface  distribution. 

Barometric  pressure  and  weather  changes. — As  has  been 
pointed  out,  the  changes  of  weather  and  the  forces  of  the  wind  are 
closely  related  to  the  changes  of  pressure  which  accompany  or  cause 
them,  and  to  the  rapidity  with  which  these  changes  take  place. 

In  dealing  with  pressure  we  may  consider  it  as  being  of  two  classes — 
periodic  and  nonperiodic.  To  the  former  belong  the  changes  that 
depend  on  the  time  of  clay  or  year  and  are  not  strictly  connected 
with  changes  of  weather,  and  to  the  latter  belong  those  that  depend 
on  the  movement  of  areas  of  high  and  low  pressure  around  the  globe; 
in  other  words,  are  dependent  on  the  movement  of  weather. 

i  Winds  of  Boston.    (McAdie.) 
50821—18 5 


66  MANUAL  OF   AEEOGKAPHY. 

All  changes  in  pressure  are  associated  with  the  changes  of  tempera- 
ture which  take  place  at  different  hours  of  the  day,  or  at  the  various 
seasons  of  the  year,  or  arise  at  different  places  on  the  earth  from 
various  causes ;  among  which  may  be  mentioned  position  with  respect 
to  latitude,  distribution  of  land  and  sea,  greater  or  less  abundance  of 
cloud  or  rain,  or  quantity  of  vapor  in  the  air. 

When  a  difference  of  pressure  is  established  air  tends  to  flow  from 
high  pressure  to  low  pressure,  not  directly  but  rather  around  the 
areas  of  high  and  low,  with  an  inclination  toward  the  center  of  the 
low.  It  must  be  kept  in  mind  that  pressure,  as  measured  by  the 
barometer,  is  determined  by  the  entire  height  of  the  atmosphere  and 
not  by  the  lower  layers  alone.  It  can  not  be  expected  that  the  dis- 
tribution of  surface  pressure  can  always  be  accounted  for  by  the 
distribution  of  surface  temperature.  Generally  speaking,  especially 
in  the  Northern  Hemisphere,  in  winter  the  barometer  is  higher  over 
the  land,  which  is  then  colder,  than  the  sea;  and  in  summer  the 
barometer  is  lower  over  the  continents  and  higher  over  the  sea, 
since  the  land  is  then  relatively  hot  and  the  sea  relatively  cold. 

The  pressure  is  relatively  lower  over  the  equatorial  region  and  over 
the  temperate  regions.  Over  the  sea  areas  just  north  of  the 
Tropic  of  Cancer  and  south  of  the  Tropic  of  Capricorn  the  pressure 
is  always  high,  as  has  been  shown  before.  Periodic  changes  of 
pressure  occur  over  such  wind  tracks  as  the  trades  and  monsoons. 

Of  the  periodic  changes  the  diurnal  variation  in  pressure,  although 
small,  must  be  considered.  This  diurnal  variation  consists  of  a 
double  oscillation  with  two  periods  of  increase  and  two  of  decrease  of 
presssure  within  24  hours;  the  barometer  rising  from  about  4  a.  m.  till 
about  10a.m.;  then  falling  until  about  4  p.  m.,  and  again  rising  until 
about  10  p.  m.,  when  it  once  more  falls  until  about  4  a.  m.  The  fore- 
noon maximum  is  commonly,  but  not  invariably,  higher  than  the 
afternoon  maximum;  and  the  former  usually  occurs  before  rather 
than  after  10  a.  m.,  while  the  latter  tends  to  be  later  rather  than 
earlier  than  10  p.  m.  The  afternoon  minimum  is,  with  rare  excep- 
tions, lower  than  the  morning  minimum  and  occurs  after  rather  than 
before  4  p.  m.1  At  sea  the  diurnal  variation  attains  its  greatest 
magnitude  within  the  Tropics  and  gradually  diminishes  in  higher 
latitudes  to  a  hardly  perceptible  quantity  in  the  Arctic  region. 

The  extent  of  the  oscillations  depends  to  a  great  extent  on  the 
range  of  daily  temperature  and  the  times  of  maximum  and  minimum 
are  influenced  by  the  times  of  sunrise  and  sunset.  The  daily  range  in 
tropical  seas  is  between  2.4  and  2.7  kilobars,  the  maximum  rise  above 
the  mean  being  somewhat  less  than  the  maximum  fall  below  it.  In 
the  British  Isles,  according  to  Shaw,  the  range  of  diurnal  change  of 

i  Barometer  Manual.     (Sir  Napier  Shaw.) 


MANUAL  OF  AEROGRAPH Y.  67 

pressure  is  only  about  0.2  to  0.7  kilobars,  so  that,  except  in  very  calm 
settled  weather,  the  daily  oscillations  can  seldom  be  recognized  in  the 
hourly  readings  of  a  barometer  during  any  given  day,  though  they 
ma  v  become  quite  apparent  in  the  means  of  such  a  period  as  a  month. 

There  is  also  an  annual  variation  of  pressure  which  is  very  evident 
in  the  Tropics  both  on  land  and  on  sea,  following  the  apparent  motion 
of  the  sun  north  and  south  of  the  equator  and  being  associated  with 
modifications  of  the  trade  winds  and  such  periodic  winds  as  the 
monsoons.  As  the  annual  variation  takes  place  gradually  it  is  not 
important  in  the  forecasting  of  daily  and  sudden  changes  of  weather. 

The  nonperiodic  changes  in  pressure,  or  those  immediately  asso- 
ciated with  weather  changes,  under  ordinary  conditions  vary  with 
latitude,  being  smallest  near  the  equator  and  increasing  as  we  recede 
from  it. 

Within  the  Tropics  the  ordinary  fluctuations  of  the  barometer,  in- 
cluding the  diurnal  variation,  seldom  exceed  10  to  14  kilobars  except 
in  event  of  one  of  those  revolving  storms  known  as  hurricanes, 
cyclones,  or  typhoons  according  to  the  part  of  the  globe  where  they 
occur,  when  the  barometer  usually  falls  much  lower,  and  in  the  lowest 
and  most  dangerous  part  of  the  depression  may  be  as  much  as  75 
kilobars  (more  than  2  inches).  Records  taken  from  a  large  number 
of  observations  in  the  equatorial  regions  covering  a  considerable 
number  of  years  show  a  range  of  pressure  of  only  14  kilobars  (0.413 
inch)  from  the  highest  reading  1020.6  kilobars  (30.138  inches) 
observed  in  July  to  the  lowest  1006.6  kilobars  (29.725  inches)  ob- 
served in  December. 

The  average  range  of  pressure  increases  with  latitude  until  it  reaches 
it>  maximum  in  the  Northern  Hemisphere  between  the  sixtieth  and 
sixty-fifth  parallels,  and  then  decreases  to  the  pole.  In  the  British 
Isle-,  according  to  Shaw,  the  average  range  in  the  course  of  a  month  is 
about  58  kilobars  (1.7  inches)  for  January  and  30  kilobars  (0.9  inch) 
for  July.  At  the  Royal  Observatory  at  Greenwich  the  highest  cor- 
rected barometric  reading  was  1048.8  kilobars  (30.972  inches)  and 
the  lowest  957.4  kilobars  (28.272  inches)  a  range  of  91.4  kilobars 
(2.70  inches).  Such  extremes  are  rather  exceptional,  as  the  pressure 
has  seldom  gone  above  1040  kilobars  (30.8  inches)  or  below  965  kilo- 
bars  (28.5  inches). 

The  following  table  taken  from  the  Barometer  Manual,  by  Sir 
Xapier  Shaw,  shows  the  mean  range  of  barometric  pressure  that  can 
be  expected  under  ordinary  conditions  and  excluding  exceptional 
storms  of  great  severity,  during  the  months  of  January  and  July  at 
different  latitudes  of  the  Northern  Hemisphere.  These  figures  are 
compiled  from  data  secured  from  all  available  authorities,  January 
and  July  being  considered  typical  winter  and  summer  months,  re- 
spectively. 


68 


MANUAL  OF   AEROGKAPHY. 


Mean  range  of  barometric  pressure. 
[In  kilobars  and  in  inches.] 


Latitude. 

January. 

July. 

January. 

July. 

Equator  to  Tropic  of  Cancer 

Kilobars. 
1    -13.  5 

Kilobars. 
7    -10 

Inches. 
0.  20-0.  40 

Inches. 
0.  20-0.  30 

Tropic  of  Cancer  to  30°  north  

13.  5-22 

10    -13.5 

.  40-  .  65 

.30-  .40 

30°  north  to  40°  north                                             

22    -42 

13.  5-20 

.  65-1.  25 

.40-  .60 

40°  north  to  50°  north 

42    -51 

20    -27 

1.  25-1.  50 

.60-  .80 

50°  north  to  60°  north  .  .               

51    -61 

27    -34 

1.  50-1.  80 

.80-1.  .00 

60°  north  to  65°  north                               

61    -58 

34 

1.  80-1.  70 

1.00 

For  the  smaller  ranges  the  assumption  that  the  variations  of  the 
height  of  the  barometer  are  of  nearly  equal  amount  on  each  side  of 
the  mean  reading  is  sufficiently  exact  for  practical  purposes. 

An  examination  of  the  pressure  records  at  the  Greenwich,  England, 
station  show  that  in  January,  the  typical  winter  month,  when  the 
fluctuations  are  greatest,  the  pressure  falls  below  the  mean  about 
five-eighths  of  the  whole  range  and  above  the  mean  three-eighths 
of  the  range;  while  in  July,  the  typical  summer  month,  when  the 
fluctuations  are  least,  the  rise  and  fall  in  the  range  is  very  nearly 
divided.  Thus,  with  an  average  barometric  reading  in  the  English 
Channel  of  1014  kilobars,  we  should  have  in  winter  with  a  range  of 
51  kilobars,  a  fall  of  32  kilobars  and  a  rise  of  19  kilobars  as  represent- 
ing the  lowest  and  highest  barometric  reading  that  may  be  expected. 
Keeping  this  range  in  mind  we  can  by  the  observation  of  the  rise  and 
fall  of  the  barometer  and  the  change  of  direction  and  force  of  the 
wind  keep  in  touch  with  abnormal  as  well  as  normal  disturbances. 

The  approach  and  passage  of  cyclonic  depressions  or  of  anticy- 
clonic  rises  give  us  our  most  marked  changes  of  weather. 

When  the  pressure  in  any  area  is  below  that  of  the  surrounding 
region,  a  cyclonic  circulation  is  formed  and  the  air  currents  will  be 
found  to  have  a  motion  around  it,  but  inclined  inward  toward  the 
center,  instead  of,  as  might  be  expected,  directly  into  the  center  of 
the  depression. 

On  the  other  hand,  when  the  pressure  is  high  in  any  area  and  de- 
creases in  the  region  surrounding  it  an  anticyclone  is  developed  and 
the  air  acquires  a  motion  around  it  but  inclined  outward.  As  has 
been  pointed  out,  the  circulation  of  air  is  counter-clockwise  around 
low-pressure  areas  and  clockwise  around  high-pressure  areas  in  the 
northern  hemisphere. 

The  steeper  the  pressure  gradients,  that  is,  the  more  rapid  the 
changes  in  pressure  the  more  rapid  will  be  the  changes  in  weather 
conditions.  C.  N.  K. 


CHAPTER  VH. 


TEMPERATURE. 


CHAPTER  VII. 


TEMPERATURE. 

The  temperature  of  a  body  is  determined  by  the  average  kinetic 
energy  of  translation  of  the  molecules  of  the  body.  Thus,  if  we  have 
a  ^ras  inclosed  in  a  cylinder  and  we  compress  it  by  thrusting  the 
piston  in,  there  will  be  an  increased  kinetic  energy  of  the  molecules, 
which  our  senses  observe  as  a  rise  in  temperature.  If  the  gas  expands 
and  thrusts  out  the  piston,  it  has  done  work,  the  average  kinetic 
energy  of  the  molecules  decreases,  and  we  have  a  lowering  of  tem- 
perature. Thus,  if  the  average  kinetic  energy  of  translation  is 
varied,  so  is  the  temperature. 

The  detection  of  gain  or  loss  of  heat,  or  temperature,  is  accom- 
plished in  several  ways,  such  as  change  of  volume,  change  of  state, 
change  of  electromotive  force,  and  change  of  electrical  resistance. 
All  of  these,  according  to  circumstances,  afford  convenient  means  of 
comparing  temperatures  of  different  objects. 

The  mercury  thermometer  and  the  alcohol  thermometer,  the  latter 
adapted  to  low  temperatures,  are  based  on  change  of  volume,  and 
the  fact  that  the  expansion  of  the  vessel  is  not  the  same  as  that  of 
the  fluid  which  it  contains,  under  equal  changes  of  temperatures. 
The  principle  of  differential  expansion  is  also  used  in  the  thermo- 
graphs or  instruments  for  the  continuous  mechanical  registration  of 
temperature.  Important  among  these  instruments  are  those  con- 
taining a  Bourdon  tube,  which  consists  of  a  curved  closed  tube  of 
oval  cross  section,  completely  filled  with  a  suitable  liquid.  The 
unequal  expansion  between  the  tube  and  the  liquid  changes  the  vol- 
ume and  that  in  turn  changes  the  curvature  of  the  tube.  Hence  by 
making  one  end  of  the  tube  fast  and  connecting  the  other  with  a 
tracing  point,  it  at  once  becomes  possible  to  obtain  on  a  moving 
record  sheet  a  complete  record  of  the  temperature  changes.  In  other 
instruments  use  is  made  of  the  unequal  expansion  of  the  two  sides 
of  a  bimetallic  strip. 

The  variation  of  electrical  resistance  with  change  of  temperature 
and  electromotive  force  at  a  thermo-j unction  both  provide  means  of 
measuring  temperature  changes  very  accurately.1  Exceptionally 
low  temperatures  down  to  near  absolute  zero  and  exceptionally 
high  ones  have  been  registered  by  these  latter  means. 

Change  of  volume  and  the  state  of  matter  as  a  means  of  detecting 
temperatures  was  made  use  of  in  developing  the  absolute  temperature 
system.  Examination  of  the  expansion  and  contraction  of  gases 

i  Humphrey's  "  Physics  of  the  Air,"  Journal  of  the  Franklin  Inst.,  Aug.,  1917. 


72  MANUAL   OF   AEEOGRAPHY. 

shows  that  if  any  permanent  gas,  for  example,  hydrogen  or  nitro- 
gen, were  to  go  on  contracting  at  the  same  rate  with  cooling  as  it 
does  at  ordinary  temperatures,  it  would  have  no  volume  and  would 
cease  to  exert  any  pressure  at  a  temperature  of  about  459.4  below 
zero  Fahrenheit,  or  273.02  centigrade.  This  temperature  is  nearly 
identical  with  the  temperature  computed  by  Lord  Kelvin  as  being 
the  minimum  below  which  it  is  impossible  to  reduce  a  body,  and  the 
temperature  to  which  a  substance  must  be  reduced  in  order  to  get 
the  full  equivalent  in  work  of  heat  supplied  to  it.  This  temperature 
is  known  as  absolute  zero.  Two  temperature  scales,  the  (Kelvin) 
absolute  and  the  Kelvin  kilograde  have  been  developed  with  this 
temperature  as  their  zero  point. 

Temperature  scales. — We  have  to-day  four  temperature  scales 
in  common  use.  The  earliest  of  these  in  use,  but  now  nearly  obsolete 
in  scientific  work,  is  the  Fahrenheit.  This  scale  is  graduated  into 
180  equal  divisions  between  the  freezing  and  boiling  point  of  dis- 
tilled water  under  the  pressure  of  an  absolute  atmosphere,  with  the 
freezing  point  32  divisions  above  the  zero  point  of  the  scale.  The 
zero  point  represents  the  lowest  temperature  of  a  mixture  of  ice 
and  salt. 

The  scale  in  common  use  on  the  Continent  of  Europe  is  the  Centi- 
grade, and  is  graduated  into  100  equal  divisions  between  the  freezing 
and  boiling  points  of  water,  under  an  absolute  atmosphere,  with  ther 
freezing  point  as  zero. 

The  absolute  scale  has  the  same  scale  divisions  as  the  Centigrade 
but  with  the  absolute  zero  as  its  zero  point.  The  freezing  point  of 
water  is  therefore  273,  with  the  boiling  point  of  water  as  373. 

The  most  recent  scale  devised  is  the  Kelvin-Kilograde,  which  has 
for  its  zero  point  the  absolute  zero  and  the  freezing  point,  under  a 
pressure  of  an  absolute  atmosphere  (a  thousand  kilobars),  1,000. 
With  this  scale  of  divisions,  the  boiling  point  of  pure  water  under  a 
pressure  of  a  thousand  kilobars  is  1 ,366.  This  scale  has  the  advantage 
of  having  smaller  and  more  suitable  scale  divisions,  no  minus  signs, 
and  no  confusion  regarding  zero  and  freezing  points. 

The  Kelvin-Kilograde,  Centigrade,  and  Fahrenheit  units  have  the 
corresponding  values  of  1 :  3.  66  :  2.  04,  respectively. 

Temperature  of  the  atmosphere. — Temperature  has  often  been 
defined  as  the  thermal  condition  of  a  body  which  determines  the 
inner  change  of  heat  between  it  and  other  bodies.  This  interchange 
of  heat  occurs  in  one  or  more  of  three  ways:  By  conduction,  by  con- 
vection, or  by  radiation.  Heat  imparted  by  conduction  is  trans- 
ferred from  particle  to  particle,  and  involves  contact  with,  or  near 
approach  to,  a  warmer  body.  Of  the  different  forms  of  matter, 
solids,  especially  metals,  are  the  best  conductors,  while  liquids  are 
better  conductors  than  gases.  Changes  in  the  temperature  of  the 
earth's  surface,  which  becomes  heated  during  the  day  and  cool 


MANUAL   OF  AEROGRAPHY.  73 

during  the  night,  are  communicated  to  the  layers  of  air  in  contact 
with  and  immediately  above  it,  by  conduction.  Heat  is  being  con- 
stantly iransmitted  by  convection  from  one  locality  to  another 
through  the  agency  of  winds  and  ocean  currents.  Heat  is  com- 
municated between  bodies  freely  exposed  to  each  other  by  means  of 
radiation.  The  communication  of  heat  by  radiation  proceeds  not 
from  one  particle  to  another  but  through  the  ether.  Radiant  heat 
is  a  form  of  energy  which  proceeds  in  straight  lines  in  all  directions 
from  a  hot  body. 

The  atmosphere  derives  its  heat,  directly  or  indirectly,  almost 
entirely  from  the  sun.  The  actual  temperature  of  the  atmosphere 
depends  not  so  much  upon  the  direct  rays  of  the  sun  as  upon  the 
conduction  and  radiation  from  the  surface  of  the  earth  heated  by 
the  sun's  rays.  The  air  is  not  heated  directly  by  sunshine,  but  the 
surface  of  the  earth  is  first  heated  by  the  sun's  rays,  and  the  air  is 
warmed  by  contact  with  the  earth,  or  by  radiation  and  convection. 

Soils  of  different  character  and  water  have  a  considerable  effect 
on  the  distribution  of  atmospheric  temperature.  Cultivated  land 
absorbs  and  radiates  heat  more  readily  than  grass  land  or  wooded 
tracts. 

The  sun's  rays  have  their  greatest  effect  in  the  Tropics  where  they 
fall  perpendicularly,  or  nearly  so,  on  the  earth's  surface.  The 
^affect  decreases  as  we  travel  toward  the  poles  and  the  rays  strike 
more  obliquely.  It  might  be  mentioned  that  hillsides  surrounding 
bodies  of  water  often  derive  a  great  deal  of  heat  by  reflection  of  the 
sun's  rays  from  the  water  surface.  Likewise  the  temperature  of 
valleys  is  often  raised  by  reflection  and  radiation  from  surrounding 
mountain  sides. 

Diurnal  variation. — In  contrast  to  the  warming  effect  due  to 
solar  radiation  and  reflection  is  the  cooling  of  the  earth's  surface. 
During  the  day  and  night  the  earth  is  parting  with  the  heat  received 
from  the  sun.  The  heat  received  from  the  sun  during  the  day  is, 
as  a  rule,  greater  than  that  which  the  earth  parts  with,  but  toward 
the  end  of  the  day  when  the  sun's  rays  fall  more  obliquely  on  the 
surface,  these  conditions  are  reversed,  and  the  loss  is  greater  than  the 
gain.  As  a  result  the  atmosphere  above  is  cooled  and  the  tempera- 
ture continues  to  fall  until  sunrise.  The  lowest  temperature  usually 
comes  at  about  the  time  of  sunrise  and  the  highest  at  from  2  to  4.30 
p.  m.  depending  upon  the  season  of  the  year.  The  maximum  occurs 
early  in  winter  and  late  in  summer.  The  average  temperature  for 
the  clay  occurs  at  about  9  a.  m.  and  8  p.  m.  The  rise  during  the 
morning  and  early  afternoon  is  sharp,  the  curve  being  convex;  while 
the  drop  during  the  afternoon  and  night  is  long  and  slow,  giving  a 
concave  curve. 

When  the  atmosphere  is  cloudy  or  overcast  the  temperature 
amplitude  is  less  than  when  it  is  clear.  Likewise  the  cooling  of  the 


74 


MANUAL   OF  AEBOGKAPHY. 


earth's  surface  and  the  air  resting  upon  it  is  not  so  great  when  the  air 
is  in  motion  as  when  there  is  a  calm.1 

Annual  variation. — A  review  of  the  temperature  records  for 
different  stations  in  the  Northern  Hemisphere  shows  that  the  aver- 
age minimum  temperature  occurs  near  the  end  of  January,  and  the 
average  maximum  temperature  during  the  last  part  of  July.  The 
annual  variation  in  temperature  varies  slightly  with  elevation, 
latitude,  and  with  the  immediate  surroundings  of  the  station. 

Fall  of  temperature  per  kilometer,  or  the  approximate  temperature  gradient  for  each  month 

of  the  year,  by  Dines. 
[In  Kelvin  kilograde  scale.] 


0-1 
Km. 

1-2 
Km. 

2-3 

Km. 

3-4 
Km. 

4-5 
Km. 

5-6 
Km. 

6-7 
Km. 

7-8 
Km. 

8-9 
Km. 

9-10  10-11 
Km.  Km. 

11-12 
Km. 

12-13 
Km. 

13-14 
Km. 

Mean 
0-9 
Km. 

January  

18.3 
18.3 
14.6 
21.9 
21.9 
21.9 
21.9 
21.9 
18.3 
14.6 
18.3 
18.3 

14.61  14.6 
18.  3   14.6 
21.91  14.6 
21.  9i  18.3 
21.9    18.3 
21.9   18.3 
18.3    18.3 
14.6!  18.3 
10.9    18.3 
18.3    18.3 
10.9   18.3 
10.9    18.3 

21.9 
21.9 
21.9 
21.9 
21.9 
21.9 
21.9 
21.9 
21.9 
21.9 
21.9 
21.9 

25.6 
25.6 
25.6 
25.6 
21.9 
21.9 
21.9 
21.9 
21.9 
21.9 
21.9 
21.9 

25.6 
21.9 
21.9 
21.9 
25.6 
25.6 
21.9 
25.  6 
25.6 
25.6 
21.9 
25.6 

21.9 
25.6 
25.6 
25.6 
25.6 
25.6 
29.3 
25.6 
25.6 
21.9 
29.3 
25.6 

25.6 
25.6 
25.6 
25.6 
21.9 
25.6 
21.9 
25.6 
21.9 
25.6 
21.9 
21.9 

21.9 
21.9 
21.9 
21.9 
25.6 
25.6 
25.6 
29.3 
29.3 
25.6 
25.6 
25.6 

14.6   10.9 
10.9    10.9 
14.6    10.9 
14.6   10.9 
18.3    14.6 
21.9    14.6 
29.3    14.6 
25.6    14.6 
25.6   18.3 
25.6   14.6 
18.3    14.6 
14.6    10.9 

-3."  7 

-7.3 
-3.7 
-3.7 
-3.7 

"&7 
3.7 
3.7 
3.7 
3.7 

3.7 
3.7 

-3."  7 

3  7 

----- 

21.-2 
21.5 
21.5 
22.7 
22.7 
23.1 
22.3 
22.7 
21.5 
21.5 
21.2 
21.2 

22.3 

February.. 

March 

April  .  .  . 

May. 

June 

-3.7 
-3.7 

"3."  7 

July  

August 

September 

3.7 
3.7 
3.7 
3.7 

"3."  7 
3.7 
3.7 

October 

November 

December  

Average  

19.4 

17.6 

17.6  21.9 

23.1 

24.  2|  25.6 

24.2 

24.9 

19.4    12.8 

-0.4 

0.7 

1.1 

It  is  evident  that  the  gradient  in  free  air  under  usual  conditions"* 
is  approximately  6°  C.    (22.3   KK)   per  kilometer.     By  a  study  of 
thermodynamics   we  find  that  if  the  atmosphere  were  dry  its  tem- 
perature would  decrease  at  an  adiabatic  rate  of  9.8°  C.  (36.6  KK)  per 
1,000  meters. 

If  an  isolated  volume  of  air  rises  or  falls  with  its  temperature 
changing  at  the  adiabatic  rate  and  the  temperature  of  the  surround- 
ing air  also  changes  at  the  adiabatic  rate,  there  will  be  no  tendency 
to  continue  the  movement.  If  the  temperature  gradient  of  the 
surrounding  air  is  less  than  adiabatic  the  isolated  mass  will  tend  to 
return  to  its  initial  level.  If  the  local  temperature  gradient  is  steeper 
than  adiabatic,  the  air  will  continue  to  rise  or  fall. 

When  the  temperature  of  the  •  atmosphere  decreases  rapidly  with 
increasing  altitude  vertical  currents  will  occur.  When  we  have 
inversion  of  temperature  aloft,  as  we  frequently  do  above  cloud 
layers,  and  the  temperature  increases  with  increase  of  altitude, 
vertical  movement  practically  ceases  as  far  up  as  the  inversion 
extends. 

Within  3J  kilometers  of  the  earth's  surface  the  rate  of  change  of 
temperature  vertically  is  very  irregular.  It  may  be  more  or  less 
than  9.8°  C.  (36  Kk)  per  kilometer,  or  there  may  even  be  a  rise  in 
temperature  with  height. 

i  Seaman's  Handbook,  M.  O.  215,  1915. 


MANUAL  OF  AEBOGBAPHY. 


75 


Influence  of  water  vapor  on  temperature  gradient. — The 
presence  of  water  vapor  in  the  atmosphere  has  a  great  effect  on 
temperature  gradients.  The  adiabatic  and  actual  gradients  up  to 
the  point  of  vapor  condensation  are,  generally  speaking,  the  same. 
From  this  point  to  the  freezing  point,  however,  the  temperature  of 
a  rising  column  of  air  will  (due  to  the  release  of  heat  of  vaporization) 
decrease  only  about  half  as  rapidly  as  before  condensation.  Owing 
to  the  large  amount  of  water  vapor  in  the  atmosphere,  at  or  near 
the  point  of  condensation,  it  has  been  found  that  the  actual  tem- 
perature gradient  follows  more  closely  the  condensation  gradient 
than  that  of  dry  air.  Therefore  we  actually  have  a  slower  decrease 
in  temperature  with  increase  in  altitude  than  would  be  expected 
under  adiabatic  conditions. 

Stratosphere  and  troposphere.— One  of  the  most  important 
findings  in  the  study  of  the  temperature  of  the  upper  air  has  been 
the  discovery  that  the  temperature  ceases  to  fall  above  a  certain 
height .  Above  this  level  the  temperature  has  been  found  to  remain 
stationary  or  even  to  rise.  Soundings  have  also  shown  that  this 
level  at  which  the  temperature  gradient  ceases  or  reverses  varies 
with  the  season  and  the  latitude.  DeBort,  who  discovered  this 
phenomenon,  called  the  upper  inversion  layer  the  stratosphere  and 
the  lower  levels  where  convection  did  occur,  the  troposphere.  He 
found  the  average  height  at  which  the  stratosphere  began  to  be 
11  kilometers.  He  also  found  that  the  height  varied  in  high  pres- 
sure and  low  pressure  areas  from  an  average  of  12.5  kilometers  in 
the  former  to  an  average  of  10  kilometers  in  the  latter.  At  the  10- 
kilometer  level  the  difference  in  pressure  between  an  average  high 
and  an  average  low  would  be  approximately  10  kilobars,  while  the 
difference  at  sea  level  is  about  70  kilobars. 

Temperatures  in  the  stratosphere  and  troposphere  for  the  different  months  (compiled  from 
the  results  of  various  ascents  made  under  the  auspices  of  the  International  Commission 
for  Scientific  Aeronautics,  by  G.  Nadler). 

[In  Kelvin  kilograde  scale.] 


Kilometers. 

Jan. 

Feb. 

Mar. 

Apr. 

May. 

June. 

July. 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

14 

790 

794 

801 

809 

812 

816 

812 

809 

801 

794 

790 

786 

13        

790 

794 

801 

809 

812 

816 

812 

809 

801 

798 

794 

790 

12 

794 

798 

801 

805 

809 

812 

812 

809 

809 

801 

798 

794 

11... 

794 

794 

794 

801 

805 

809 

812 

812 

809 

805 

801 

798 

10... 

805 

805 

805 

812 

820 

823 

827 

827 

827 

820 

816 

809 

9 

S20 

816 

820 

827 

838 

845 

856 

853 

853 

845 

834 

823 

8 

842 

838 

842 

849 

864 

871 

882 

882 

882 

871 

860 

849 

7 

M,7 

864 

867 

875 

886 

897 

904 

908 

904 

897 

882 

871 

6!" 

m 

889 

893 

900 

911 

922 

933 

933 

930 

919 

911 

897 

5 

914 

911 

914 

922 

927 

949 

955 

977 

955 

944 

933 

922 

4... 

940 

927 

940 

949 

959 

970 

977 

981 

977 

966 

955 

944 

3 

962 

959 

962 

970 

981 

:>'.>  > 

1,000 

1,004 

992 

988 

977 

966 

2 

977 

973 

977 

988 

/  000 

1  010 

1  017 

1  021 

1  010 

1  006 

996 

984 

1. 

at 

992 

1,000 

1,010 

1,021 

1,032 

1,036 

1,036 

1,028 

1,021 

1,006 

996 

Ground 

1,010 

1,010 

1  014 

1,032 

1  043 

1  054 

1  058 

1,058 

1,047 

1,036 

1,024 

1,014 

The  temperatures  in  bold  face  indicate  the  lowest  reached  and  the  apparent  beginning  of  the  stratosphere 
or  isothermal  layer. 

The  temperatures  in  italics  show  the  freezing  temperature,  which  descends  to  sea  level  in  latitude  63° 
north  and  south,  but  which  at  the  Equator,  is  more  than  5  kilometers  above  sea  level. 


76  MANUAL   OF   AEROGRAPHY. 

The  accompanying  table,  which  is  a  review  of  the  results  of  various 
upper  air  soundings  made  over  the  globe  under  the  auspices  of  the 
International  Commission  for  Scientific  Aeronautics,  gives  a  very  good 
idea  of  the  temperatures  that  can  be  expected  at  different  levels  and 
also  the  changes  of  temperature  with  seasons  at  the  different  levels. 
However,  the  height  of  the  stratosphere  over  the  Equator  has  been 
found  to  be  much  greater  than  over  the  temperate  regions,  and  also 
the  lowest  temperatures  in  the  upper  ah*  have  been  recorded  above 
the  Equator.  From  the  soundings  made  at  Batavia,  Java,  and  other 
points  on  the  Equator  it  was  found  that  the  mean  height  of  the  strato- 
sphere is  just  under  17  kilometers  with  an  average  temperature  of 
187  A. 

The  important  features  of  the  stratosphere  are:  1.  The  great  ele- 
vation and  lowered  temperature  over  the  tropical  region.  2.  That 
the  height  varies  with  the  seasons  and  the  pressure  distribution. 

Cause  of  stratosphere. — While  there  are  many  explanations  for 
the  existence  of  the  stratosphere,  radiation  seems  to  be  the  basis  for 
most  of  the  explanations.  According  to  Gold,  radiation  seems  to  have 
a  heating  effect  above  a  level  where  the  pressure  is  250  kilobars,  while 
below  that  level  it  has  a  cooling  effect.  He  assumes  that  convec- 
tive  temperature  equilibrium  exists,  and  that  there  is  the  usual 
decrease  of  water  vapor  with  elevation.  Braak  contends  that  the 
very  low  temperatures  in  the  upper  limits  of  the  troposphere  in  trop- 
ical regions  must  have  some  connection  with  the  rising  air  currents  of 
the  general  circulation  which  disturb  the  distribution  of  temperature 
as  determined  by  radiation  and  absorption  and  shift  the  troposphere 
to  greater  heights.  The  base  of  the  cirrus  clouds,  according  to  Braak, 
represents  fairly  well  the  height  of  the  hypothetical  dividing  surface 
between  the  cooling  and  heating  effect  of  radiation  for  moist  air.1 

C.  N.  K. 

i "  Principles  of  Aerography,"  McAdie. 


CHAPTER  Vffl. 


WATER  VAPOR. 


77 


CHAPTER  VIII. 


WATER  VAPOR.1 


The  vapor  of  water  is  one  of  the  most  important  constituents  of 
the  atmosphere,  although  the  total  amount  present  is  small.  It 
exi>ts  in  various  forms,  as  invisible  vapor,  condensed  vapor,  liquid 
and  solid. 

Thus  we  have  vapor  masses  which  can  not  be  differentiated  by 
the  eye  from  the  surrounding  air  and  can  only  be  detected  by  their 
effort  upon  the  noodle  of  an  electrometer.  Again,  the  vapor  is  visi- 
ble, as  in  the  clouds,  mists,  and  fogs,  and  finally  as  precipitated  or 
cr\stallize<l  water  in  dow,  glaze,  rain,  hail,  frost,  and  rime. 

At  the  earth's  surface  water  vapor,  according  to  Humphrey's,  sup- 
plies 1.2  per  cent  of  the  total  number  of  gas  molecules  present.2 

The  so-called  tension  of  water  vapor  or  pressure  per  unit  area  is 
therefore  small  compared  with  the  pressure  of  the  other  gases  or  the 
combined  aerostatic  pressure.  The  ratio,  if  expressed  in  millimeters, 
would  be  about  4/760  at  a  temperature  of  1000  kk.  (freezing)  and 
27/1000  at  1100  kk.  under  conditions  of  saturation.  If  the  pressure 
is  to  be  expressed  in  units  of  force,  then  the  following  ratio  may  be 
used:  Saturation  vapor  pressure  at  temperature  1000  kk.  will  be  6 
kilobars,  while  the  aerostatic  pressure  is  1016  kilobars. 

A  better  way,  however,  is  to  define  absolute  humidity  as  the  mass 
of  water  vapor  per  unit  volume,  preferably,  in  grams  per  cubic  meter 
of  space. 

We  then  begin  to  clearly  comprehend  that  the  air  and  the  water 
vapor  exist  independently  of  each  other,  and  furthermore  that  as 
temperature  falls  the  weight  of  dry  air  per  unit  volume  increases, 
because  of  greater  density,  while  the  weight  of  water  vapor  decreases. 

Relative  humidity  is  simply  the  percentage  of  the  absolute  humidity 
existing  at  any  given  time.  It  is  the  ratio  of  the  actual  pressure  of 
the  vapor  to  the  saturation  pressure  for  the  given  temperature.  Or 
again  it  may  be  defined  as  the  weight  of  the  water  vapor  per  unit 
volume  present  to  the  weight  when  a  condition  of  saturation  prevails, 
saturation  being  100  per  cent.  Thus,  in  the  example  given  above  if 
the  saturation  weight  is  expressed  in  force  units  as  6  kilobars  then  3 
kilobars  would  represent  50  per  cent  relative  humidity. 


1  Principles  of  Aerography,  McAdie. 

2  Physicsof  the  Air,  Journ.  Franklin  Inst..  Sept.,  1917,  p.  389. 


79 


80  MANUAL  OF   AEROGRAPH Y. 

Unless  the  temperature  be  given,  a  statement  of  relative  humidity 
has  no  especial  value  as  it  is  plain  that  a  percentage  of  a  force  or 
weight  which  varies  with  temperature,  applies  only  to  the  given 
value.  Thus,  in  many  climatological  publications,  tables  of  relative 
humidity  are  given,  for  comparative  purposes,  but  such  tables  defeat 
their  own  purpose,  unless  in  each  instance  the  temperature  is  stated. 

When  the  actual  and  sensible  temperatures  are  the  same,  we  have 
the  temperature  of  saturation.  When  the  sensible  or  wet  bulb  reads 
lower  than  the  actual  or  dry  bulb,  the  temperature  indicated  by  the 
wet  bulb  is  known  as  the  temperature  of  evaporation. 

The  temperature  of  saturation  is  usually  called  the  dew  point. 
The  term  "saturation  deficit7'  is  the  complement  of  the  relative 
humidity  or  the  weight  per  unit  volume  needed  to  equal  the  saturation 
weight. 

The  various  physical  processes  connected  with  the  distribution  of 
water  vapor  are  evaporation,  condensation  and  precipitation. 

Water  vapor  may  be  recorded  by  direct  observation  of  the  increase 
in  weight  of  a  hygroscopic  material  properly  exposed,  such  as  pumice 
coated  with  chemically  pure  sulphuric  acid,  phosphorus-pentoxide 
or  other  substances. 

The  usual  method  of  determining  humidity,  however,  is  by  means 
of  a  psychrometer  (the  word  means  to  measure  the  chilliness,  or  fall 
in  temperature  due  to  evaporation)  which  is  a  combination  dry  and 
wet  bulb  thermometer. 

Glaisher's  hygrometric  tables,  which  are  used  in  Great  Britain  and 
most  English  speaking  countries,  are  based  on  the  determination  of 
the  dew  point  or  temperature  of  saturation.  In  these  tables  there  is 
given  the  temperature  of  the  dry  bulb  or  what  is  commonly  called 
the  actual  temperature,  the  temperature  of  the  wet  bulb  commonly 
called  the  sensible  temperature,  and  the  temperature  of  saturation 
commonly  called  the  dew-point. 

The  formula  used  is — 

Ta-Td=C(Ta-Tw) 
in  which — 

Ta  =  actual  temperature, 

Tw  =  evaporation  temperature, 

Td  =saturation  temperature; 

and  C  =the  Glaisher  factor  as  determined  from  many  observations. 

The  value  of  C  at  1000  kk  (273  k)  is  approximately  3.3;  at  1010  kk 
(276  k)  is  2.4;  at  1025  kk  (280k)  is  2.2;  and  at  1045  kk  (285x)  is  2.0; 
at  1100  kk  (300K)  is  1.7. 

In  countries  other  than  those  mentioned  above,  formulae  based 
on  Regnault's  experiments  are  used — the  most  general  form  of  the 
equation  being — 

e"  =  e'-CP  (Ta-Tw) 


MANUAL    OF    AEROGRAP1IV.  81 

Where— 

e"  =  vapor  pressure  at  saturation. 

e'  =  vapor  pressure  at  evaporation. 

C  =  a  constant. 

P  =  aerostatic  pressure  =  1,000  Kb. 

The  values  of  C  vary  with  the  rapidity  with  which  the  wet  bulb  is 
slung  or  whirled.  In  other  words,  the  evaporation  is  a  function  of 
the  volume  of  air  passing  over  the  evaporating  surface.  At  ordinary 
pressures  the  value  of  CP  is  0.66.  If  a  ventilated  psychrometer  is 
used  the  value  of  C  is  about  0.00066.  When  the  temperature  of  the 
wet  bulb  is  below  the  freezing  point  different  values  of  C  are 
used,  depending  upon  the  condition  of  the  coating  of  the  wet  bulb 
whether  it  is  ice  or  subcooled  water.  If  the  bulb  is  covered  with 
water  and  there  is  no  air  motion,  the  value  of  C  is  0.0012.  If  the 
bulb  is  covered  with  water  and  a  moderate  movement  of  the  air  pre- 
vails, the  value  is  0.0008,  and  if  well  ventilated  the  value  rises  to 
0.0006.  If  the  bulb  has  a  coating  of  ice,  and  there  is  no  air  move- 
ment, the  value  of  C  is  0.0011;  in  light  wind,  it  is  0.0007,  and  in 
strong  wind,  0.0006. 

If  Assmann's  aspiration  psychrometer  is  used,  the  value  of  the 
constant  is  0.00066.  In  the  United  States  the  tables  used  are  those 
compiled  by  Marvin  from  Ferrel's  formula— 

e  =  e'  -0.000367  P(T  -T,)  (\  +  -r 


P(T  -TJ  ( 


The  psychrometer  is  whirled  and  the  air  movement  is  therefore 
about  that  of  a  strong  wind.  No  device,  however,  has  been  intro- 
duced to  record  the  approximate  velocity,  except  at  Blue  Hill  observ- 
atory. 

The  following  remarks  are  taken  from  "  Psychrometer  Tables  for 
Obtaining  the  Vapor  Pressure,  Relative  Humidity  and  Temperature 
of  the  Dew  Point,  from  readings  of  the  wet  and  dry  bulb  thermome- 
|»s,"  by  C.  F.  Marvin,  W.  B.  235,  Government  Printing  Office,  1913: 

The  weight  of  aqueous  vapor  (absolute  humidity}. — The  weight  of  a  cubic  foot  of 
aqueous  vapor  at  different  temperatures  and  percentages  of  saturation  is  sometimes 
called  the  absolute  humidity. 

Saturated  aqueous  vapor  is  but  little  more  than  half  as  heavy  as  the  same  volume 
of  dry  air  under  like  conditions  of  temperature  and  pressure.  In  all  ordinary  com- 
putations it  is  assumed  that  the  expansion  and  contraction  of  partially  saturated 
aqueous  vapor  is  in  accordance  with  the  same  laws  as  apply  to  air  and  ordinary  gases, 
which  do  not  easily  condense  to  the  liquid  state. 

The  adopted  density  of  saturated  aqueous  vapor  is  not  determined  directly  from 
experiment,  but  is  deduced  theoretically  from  the  observed  fact  that  two  volumes 
of  hydrogen  and  one  of  oxygen  combine  to  produce  two  volumes  of  water  vapor. 

The  weights  of  unit  volumes  of  hydrogen,  oxygen,  and  dry  air  are  accurately  known, 
from  which  the  specific  gravity  of  aqueous  vapor  is  found  to  be  0.6221. 

50821—18 6 


82  MANUAL   OF   AEROGBAPHY. 

If  English  units  of  temperature,  pressure  and  weight  are  used,  we  find  the  weight 
of  a  cubic  foot  of  saturated  aqueous  vapor  in  grains  is: 


W= 11.7449 


1+0.002037  (£-32 


In  reducing  psychrometric  observations,  regard  should  be  had  to  the  atmospheric 
pressure  at  the  time,  and  results  deduced  from  the  tables  based  on  a  pressure  nearest 
that  observed.  Interpolation  for  intermediate  pressures  need  not  be  made,  and 
when  the  pressure  is  not  observed,  an  approximate  value,  known  to  be  appropriate 
to  the  particular  elevation  of  the  point  of  observation,  may  be  employed. 

The  psychrometric  observations  made  at  Weather  Bureau  stations  will  be  reduced 
by  means  of  the  tables  based  on  an  air  pressure  which  is  numerically  nearest  the 
average  or  normal  station  pressure. 

The  temperatures  t  and  t'  of  the  wet  and  dry  bulb  thermometers  will  be  read,  and 
the  difference  t  —  tf  computed  to  the  nearest  tenth  of  a  degree.  It  is  desired  that  the 
dew-point  especially  be  taken  out  to  the  nearest  whole  degree,  and  the  tables  have 
been  expanded  with  a  view  to  obviating  difficult  interpolations.  In  some  cases, 
however,  double  interpolations  must  be  considered  but  the  proper  result  can  often 
be  obtained  by  simple  inspection.  When  the  air  is  very  dry,  however,  a  careful 
calculation  is  necessary. 

An  examination  of  the  dew-point  tables  especially  will  show  that  diagonal  lines 
exist,  inclining  downward,  and  to  the  right,  along  which  the  tabulated  values  of  the 
dew-points  are  constant  or  change  very  little.  As  a  result  of  this  circumstance,  when 
the  observed  values  of  air  temperature  and  t  —  t'  fall  even  roughly  midway  between 
the  values  given  in  the  arguments  of  the  table,  double  interpolation  will,  in  general 
not  be  required,  as  the  correct  result  will  be  obtained  by  dropping  both  intermediate 
fractions,  even  where  they  exceed  half  the  interval — that  is,  take  out  the  dew-point 
corresponding  to  the  arguments  next  lower  than  the  air  temperature  and  t—t'  observed. 

When  one  of  the  observed  quantities  is  quite  near  a  tabulated  value,  the  latter  will 
be  used,  and  the  interpolation,  if  any  is  required,  based  on  the  other  quantity  only. 

When  the  air  is  very  dry  the  successive  values  in  the  table  differ  so  much  that 
carefully  calculated  interpolations  will  often  be  required. 

The  following  example  of  the  use  of  the  tables  illustrates  how  the  foregoing  principles 
are  applied : 

Example.— Mr  pressure  27.0  inches. 

Air  temperature,  £=75.0°  F. 

Depression  of  the  wet  bulb  (t-t/)=^.b°. 

In  this  case  the  table  for  27  inches  air  pressure  should  be  used,  and  we  find,  opposite 
75°  in  column  5.5°,  dew-point=67°  F. 

Opposite  67°,  under  the  column  headed  vapor  pressure,  we  find  vapor  pressure, 
e=0.661. 

Finally,  on  page  9,  opposite  75°,  in  column  5.5°,  we  find  relative  humidity =77 
per  cent. 


.MAN  UAL    (»F    AEROGRAPH  V. 


83 


TABLE  I. —  '/'<•//•/«•/•"/»»•<  <>j '  ,1,  //  -point  in  degrees  Fahrenheit. 
[Pressure=27.0  inches.) 


Air 
tem- 
pera- 
ture. 

/ 

Vapor 
pres- 
sure. 
e 

Depression  of  wet-bulb  thermometer  (i-t). 

.2 

.4 

.6 

, 

1.0 

1.2 

1.4 

1.6 

1.8 

2.0 

2.2 

2.4 

2.6 

2.8 

3.0 

0 

0.03S3 

-  1 

-  2 

-  4 

-  5 

-  7 

-  8 

-10 

-12 

-15 

-17 

-20 

-23 

-28 

-33 

-42 

+  1  1 

403 

±  0 

-  1 

-  3 

-  4 

-  5 

-  7 

-  9 

-11 

-13 

-15 

-17 

-20 

-24 

-29 

-34 

2 

423 

-1-  1 

±  0 

-  2 

-  3 

-  4 

-  6 

-  7 

-  9 

-11 

-13 

-IS 

-is 

-a 

-25 

-29 

3 

444 

2 

-I-  I 

-   1 

-  2 

-  3 

-  4 

-  6 

-  7 

-  9 

-11 

-13 

-16 

-18 

-22 

-26 

r  * 

467 

3 

2 

+  1 

-  1 

-  2 

-  a 

-  4 

-  6 

-  8 

-  9 

-11 

-14 

-16 

-19 

-22 

.  0491 

4 

3 

2 

+  1 

-   1 

-  2 

-  3 

-  .5 

-  6 

-  8 

-10 

-12 

-14 

-16 

-19 

6               515 

5 

4 

3           2 

+   1 

-  1 

-  2 

-  3 

-  5 

-  6 

-  8 

-10 

-12 

-14 

-17 

7               542 

6 

5 

4           3 

•2 

+  1 

-  1 

-  2 

-   3 

-  5 

-  6 

-  8 

-10 

-12 

-14 

s               570 

7 

6 

5           4 

3 

2 

+  1-1 

-  2 

-  3 

-  5 

-  6 

-  8 

-10 

-12 

9               600 

8 

7 

6           5 

4 

3 

2    +   1 

-  1 

-  2 

g 

-  5 

-  6 

-  8 

-10 

10            .0631 

9 

8 

7           6 

4 

3          2 

+  1 

±  0 

-  2 

-  3 

—  4 

-  6 

-  8 

11 

665 

10 

9 

9           8 

7 

6 

5         4 

2 

+  1 

±  0 

-  2 

-  3 

-  4 

-  6 

12 

r»m 

oyy 

11 

10 

10           9 

x 

7 

6         5 

4 

3 

+  1 

±  o 

-  1 

-  3 

-  4 

u 

735 

12 

11 

11 

10 

9 

8 

7         6 

.5 

4 

3 

+  2 

±  0 

-  1 

-  2 

14               772  ; 

13 

13 

12 

11 

10 

9 

8         8 

7         6 

4 

3 

+  2 

+  1 

-  2 

15            .0810 

14 

14 

13 

12 

11 

10        10         9 

8         7 

6 

5 

4 

2 

+  1 

16 

850  i 

15 

15 

14 

13 

12 

12       11 

10 

9         8 

7 

6 

5 

4 

3 

17 

m 

16 

16 

15 

14 

13 

13       12       11 

10 

9 

9 

'8 

7 

6 

5 

18 

933 

17 

17 

16 

15 

15 

14        13       12 

11 

11 

10 

9 

8 

7 

6 

19            .  0979 

18 

18 

17 

16 

16 

15        14 

13 

13 

12 

11 

10 

9 

8 

8 

20            .  1026 

19 

19 

18 

17 

17 

16       15       15 

14 

13 

12 

11 

11 

10 

9 

TABLE  II. —  Temperature  of  dew- point  in  degrees  Fahrenheit. 
[Pressure =27.0  inches/.  • 


Air 
tem- 
pera- 
ture. 
t 

Depression  of  wet-bulb  thermometer  (t  —  /'). 

3.2 

3.4 

3.6        3.8 

4.0 

4.2 

4.4 

4.6 

4.8 

5.0        5.2 

u 

5.6 

5.8 

6.0 

0 

-58 

+  1 

-44 

2 

—36 
-30 

0/5 

—  47 
-37 

01 

-50 

on            so 

(i.2 

M 

6.6 

6.8 

7.0 

5 

-23 

-27 

-32 

-40 

-56 

16 

17 

ii'j 

48 

6 

8 

-20 

-17 
-14 

-23 
-20 
-17 

-27 
-23 
-20 

-33 
-28 
-24 

-42 

-34 

-28 

-60 
-44 
-34 

-4.5 

18 

-27 
-21 

-32 
-25 

-43 
-30 

-39 

-56 

9 

-12 

-14 

-17 

-20 

-24 

-29 

-34 

-45 

20 

-16 

-19 

-23 

-29 

-36 

11 

-  8 

—  12 
-10 

-12 

—  17 
-14 

-17 

-20 

—28 
-24 

-28 

—35     —46 

12 

-  6 

-  7 

-  9 

-11 

-14 

-16 

-19 

-23 

-28     -34 

-45 

13 

-  4 

-  5 

-  7 

-  9 

-11 

-13 

-16 

-19 

-23      -27 

-33 

AA 

14 

-  2 

-  3 

-  5 

-  7 

-  9 

-11 

-13 

-16 

18 

-22 

-26 

-32 

-42 

-60 

15 
16 
17 

±0 
-1-  2 
3 

-  2 

i   0 

+  2 

-  3 
-  1 

+  1 

-  5 
-  3 

-  6 
-  4 
-  2 

-  8 
—  fi 
-  3 

-10 
-  7 
-  5 

-12 
-  9 

-  7 

-15 
-12 
—  9 

-18 
-14 
-11 

-21 
-17 
-13 

-26 
-20 
-16 

-31 
-24 
—  19 

-40 
-30 
—23 

-57 
-38 
—28 

18 
19 

5 
6 

4 
5 

1 

4 

+  I 
3 

±  0 

+  2 

±  0 

-  3 
-  1 

-  4 
-  2 

—  6 

-  8 
-  6 

-10 

-  7 

-13 
-  9 

-15 
-12 

-18 
-14 

-22 
-17 

20 

8 

7 

I 

"' 

4 

+  2 

-     1 

±  0 

-  2 

-  3 

-  5 

-  7 

-  9 

-11 

-13 

84 


MANUAL  OF   AEEOGRAPHY. 


TABLE  III. —  Temperature  of  dew-point  in  degrees  Fahrenheit. 
|Pressure=27.0  inches.] 


Air 
tem- 
pera- 
ture. 
t 

Vapor 
pres- 
sure. 
e 

Depression  of  wet-bulb  thermometer  (t—f). 

.5 

1.0 

1.5 

2.0 

2.5 

3.0 

3.5 

4.0 

4.5  5.0 

5.5 

6.0 

6-5 

1 
7.0 

7.5 

8.0 

20 

0.103 

19 

17 

15 

13 

11 

9 

6 

4 

±0-3 

-  8 

-13 

-21 

-36 

21 

.108 

20 

18 

16 

14 

12 

10 

8 

5 

+  2-1 

-  5 

-10 

-17 

-26 

-50 

22 

.113 

21 

19 

17 

15 

14 

12 

9 

7 

4  |+  1 

-  3 

-  7 

-13 

-20 

-33 

23 

.118 

22 

20 

18 

17 

15 

13 

11 

9 

6    3 

—  1 

-  4 

-  9 

-16 

-25 

-47 

24 

.124 

23 

21 

20 

18 

16 

14 

12 

10 

8    5 

+  2 

-  2 

-  6 

-12 

-19 

-31 

25 

0.130 

24 

22 

21 

19 

17 

16 

14 

12 

9    7 

4 

±  o 

-  3 

-  8 

-14 

-23 

26 

.136 

25 

23 

22 

20 

19  ! 

17 

15 

13 

11    9 

6 

+  3 

-  1 

-  5 

-10 

-17 

27 

.143 

26 

24 

23 

22 

20  ; 

18 

16 

14 

12 

10 

8  ;   5 

+  1 

-  2 

—  7 

-13 

28 

.150 

27 

25 

24 

23 

21 

20 

18 

16 

14 

12 

10    7 

4 

±  0 

-  4 

-  9 

29 

.157 

28 

27 

25 

24 

22 

21 

19 

17 

15 

13 

11    9 

6 

+  3 

—  1 

—  5 

30 

0.164 

29 

28 

26 

25 

24 

22 

20 

19 

17 

15 

13 

11 

8 

5 

+  2 

o 

31 

.172 

30 

29 

27 

26 

25 

23 

22 

20 

18 

17 

14 

12 

10 

7 

4 

+  1 

32 

.180 

31 

30 

28 

27 

26 

24 

23 

21 

20 

18 

16   14 

12 

9 

7 

3 

33 

.187 

32 

31 

30. 

28 

27 

26 

24 

23 

21 

20 

18   16 

14 

11 

9 

6 

34 

.195 

33 

32 

31 

29 

28 

27 

26 

24 

3 

21 

19 

17 

15 

13 

11 

8 

35 

0.203 

34 

33 

32 

30 

29 

28 

27 

25 

24 

22 

21 

19 

17 

15 

13 

10 

36 

.211 

35 

34 

33 

31 

30 

29 

28 

27 

25 

24 

22 

20 

19 

17 

14 

12 

37 

.219 

36 

35 

34 

32 

31 

30 

29 

28 

26 

25 

24 

22 

20 

18 

16 

14 

38 

.228 

37 

36 

35 

33 

32 

31 

30 

29 

27 

26 

25 

23 

22 

20 

18 

16 

39 

.237 

38 

37 

36 

35 

33 

32 

31 

30 

29 

27 

26 

25 

23 

22 

20 

18 

40 

0.247 

39 

38 

37 

36 

34 

33 

32 

31 

30 

29 

27 

26 

24 

23 

21 

20 

41 

.256 

40 

39 

38 

37 

36 

34 

33 

32 

31 

30 

28 

27 

26 

24 

23 

21 

42 

.266 

41 

40 

39 

38 

37 

36 

34 

33 

32 

31 

29 

28 

27 

26 

24 

23 

43 

.277 

42 

41 

40 

39 

38 

37 

36 

34 

33 

32 

31 

29 

28 

27 

25 

24 

44 

.287 

43 

42 

41 

40 

39 

38 

37 

35 

34 

33 

32 

31 

29 

28 

27 

25 

45 

0.298 

44 

43 

42 

41 

40 

39 

38 

37 

35 

34 

33 

32 

31 

29 

28 

27 

46 

.310 

45 

44, 

43 

42 

41 

40 

39 

38 

37 

35 

34 

33 

32 

31 

29 

28 

47 

.322 

46 

45 

44 

43 

42 

41 

40 

39 

38 

37 

35 

34 

33 

32 

30 

29 

48 

.334 

47 

46 

45 

44 

43 

42 

41 

40 

39 

38 

37 

35 

34 

33 

32 

30 

49 

.347 

48 

47 

46 

45 

44 

43 

42 

41 

40 

39 

38 

37 

35 

34 

33 

32 

1 

50 

0.360 

49 

48 

47 

46 

45 

44 

43 

42 

41 

40 

39 

38 

37 

36 

34 

33 

51 

.373 

50 

49 

48 

47 

46 

45 

44 

43 

42 

41 

40 

39 

38 

37 

36 

34 

52 

.387 

51 

50 

49 

48 

47 

47 

46 

45 

44 

43 

42 

40 

39 

38 

37 

36 

53 

.402 

52 

51 

50 

50 

49 

48 

47 

46 

45 

44 

43 

42 

40 

39 

38 

37 

54 

.417 

53 

52 

51 

51 

50 

49 

48 

47 

46 

45 

44 

43 

42 

41 

39 

38 

55 

0.432 

54 

53 

52 

52 

51 

50 

49 

48 

47 

46 

45 

44 

43 

42 

41 

40 

56 

.448 

55 

54 

54 

53 

52 

51 

50 

49 

48 

47 

46 

45 

44 

43 

42 

41 

57 

.465 

56 

55 

55 

54 

53 

52 

51 

50 

49 

48 

47 

46 

45 

44 

43 

42 

58 

.482 

57 

56 

56 

55 

54 

53 

52 

51 

50 

49 

48 

47 

46 

45 

44 

43 

59 

.499 

58 

57 

57 

56 

55 

54 

53 

52 

51 

50 

50 

49 

48 

47 

46 

45 

60 

0.517 

59 

58 

58 

57 

56 

55 

54 

53 

53 

52 

51 

50 

49 

48 

47 

46 

61 

.536 

60 

59 

59 

58 

57 

56 

55 

54 

54 

53 

52 

51 

50 

49 

48 

47 

62 

.555 

61 

60 

60 

59 

58 

57 

56 

56 

55 

54 

53 

52 

51 

50 

49 

48 

63 

.575 

62 

61 

61 

60 

59 

58 

57 

57 

56 

55 

54 

53 

52 

51 

50 

49 

64 

.595 

63 

62 

62 

61 

60 

59 

59 

58 

57 

56 

55 

54 

53 

53 

52 

51 

65 

0.616 

64 

64 

63 

62 

61 

60 

60 

59 

58 

57 

56 

55 

54 

54 

53 

52 

66 

.638 

65 

65 

64 

63 

62 

61 

61 

60 

59 

58 

57 

57 

56 

55 

54 

53 

67 

.661 

66 

66 

65 

64 

63 

62 

62 

61 

60 

59 

58 

58 

57 

56 

55 

54 

68 

.684 

67 

67 

66 

65 

64 

64 

63 

62 

61 

60 

60 

59 

58 

57 

56 

55 

69 

.707 

68 

68 

67 

66 

65 

65 

64 

63 

62 

62 

61 

60 

59 

58 

57 

56 

70 

0.732 

69 

69 

68 

67 

66 

66 

65 

64 

63 

63 

62 

61 

60 

59 

58 

58 

71 

.757 

70 

70 

69 

68 

67 

67 

66 

65 

64 

64 

63 

62 

61 

60 

60 

59 

72 

.783 

71 

71 

70 

69 

68 

68 

67 

66 

65 

65 

64 

63 

62 

62 

61 

60 

73 

.810 

72 

72 

71 

70 

69 

69 

68 

67 

66 

66 

65 

64 

63 

63 

62 

61 

74 

.838 

73 

73 

72 

71 

71 

70 

69 

68 

68 

67 

66 

65 

64 

64 

63 

62 

75 

0.866 

74 

74 

73 

72 

72 

71 

70 

69 

69 

68 

67 

66 

66 

65 

64 

63 

76 

.896 

75 

75 

74 

73 

73 

72 

71 

70 

70 

69 

68 

67 

67 

66 

65 

64 

77 

.926 

76 

76 

75 

74 

74 

73 

72 

72 

71 

70 

69 

69 

68 

67 

66 

65 

78 

.957 

77 

77 

76 

75 

75 

74 

73 

73 

72 

71 

70 

70 

69 

68 

67 

67 

79 

0.989 

78 

78 

77 

76 

76 

75 

74 

74 

73 

72 

71 

71 

70 

69 

68 

68 

>-0 

1.022 

79 

79 

78 

77 

77 

76 

75 

75 

74 

73 

72 

72 

71 

70 

70 

69 

MANUAL    OF    AEROGRAPHY. 


85 


TABLE  IY, -^Temperature  of  dew-point  hi  degrees  FahrenJnit. 
(Pressure*- 27.0  inches.) 


Air 
temp. 

' 

Vapor 
press. 

' 

Depression  of  wet-bulb  thermometer  (t-f). 

8.5 

9.0 

9.5 

10.0    10.5 

11.0 

11.5 

12.0    12.5 

13.0 

HJ 

14.0 

14.5 

15.0 

15.5 

16.0 

25         0.  130 

-41 

26  |         .136 

-28 

27            .  143 

-20    -36 

28  .•         .150 

-15    -25 

-48 

29            .157 

-11 

-H 

-30 

30         0.164 

-  7 

-13 

-21 

—39 

:U            .172 

-  3 

-  8 

-15 

-25    -52 

32  I         .180 

±  0 

-    I 

-10 

-18    -30 

33            .187 

+  3 

-  1 

-  6 

-12    -20 

-36 

134 

.195 

5 

+  2 

-  2 

-  7 

-14 

-24 

-47 

35 

0.203 

8 

I 

+  1 

-  3 

-  9 

-16 

-28 

36 

.211 

10 

7 

4 

±  o 

-  5 

-10 

-18   -33 

37 

.219 

12 

9 

6 

+  3-1 

-  6 

-12    -21    -41 

> 

.228 

14 

11 

9 

+  2 

-  2 

-  7    -14    -24 

-51 

39 

.237 

16 

13 

11 

8 

5 

+  1 

-3-8    -16 

-27 

40 

0.247 

18 

15 

13 

u 

8 

5 

+  1    -  4    -10 

-17 

-32 

41    1        .256 

19 

17 

15 

13 

10 

7 

4 

±0-5 

-11 

-20 

-36 

i'2            .  266 

21 

19 

17 

15 

13 

ID 

7 

+  3-1 

-  6 

-12 

-22 

-44 

43           .  277 

a 

21 

19 

17 

15 

12 

10 

7+3 

1 

-  6 

-14 

-24 

-55 

44 

.287 

24 

22 

20 

18 

16 

14 

12 

9 

6 

+  2 

-  2 

-  7 

-15 

-27 

45 

0.298 

25 

24 

22 

20 

18 

16 

14        11 

9 

6 

-1-  2 

-  3 

-  8 

-16 

-30 

46 

.310 

X 

25 

23 

22 

20 

1^ 

'  1'i        13        11 

8 

5 

+  1 

-  3 

-  9 

-18 

-33 

47 

.322 

28 

26 

25 

23 

22 

20 

18  |     15       13 

11 

8 

5 

+  1 

-  4 

-10 

-19 

;- 

.334 

a 

28 

26       25 

23 

21 

20 

17       15 

13 

10 

8 

4 

±  0 

-  5 

-11 

49 

.347 

30 

29 

2s 

26 

25 

• 

21 

19       17 

15 

13 

10 

7 

+  4 

-  1 

-  5 

50 

0.360 

32 

30 

29 

28 

26 

25 

23 

21       19 

17 

15 

13 

10 

7 

+  3 

-  1 

51 

.373 

33 

32 

30       29 

28 

M 

25 

23       21 

19 

17 

15 

12 

10 

6 

+  3 

52 

.387 

34 

33 

32       30       29 

28 

26 

24       23 

21 

19 

17 

15 

12 

9 

6 

53 

.402 

36 

34 

33       32 

30 

M 

28 

26       24 

23 

21 

19 

17 

14 

12 

9 

54 

.417 

37 

36 

34 

33 

32 

30 

29 

28       26 

24 

23 

21 

19 

17 

14 

12 

55 

0.432 

38 

37 

36 

34 

33 

32 

30 

29       28 

26 

24 

23 

21 

19 

17 

14 

56 

.448 

40 

38 

37 

36 

34 

33 

32 

30       29 

28 

26 

24 

23 

21 

19 

17 

57 

.465 

41 

40 

39 

37 

36 

96 

33 

32       31 

29 

28 

26 

24 

23 

21 

19 

58 

.482 

42 

41 

40 

39 

37 

:\<\ 

35 

33       32 

31 

28 

28 

26 

24 

23 

21 

59 

.499 

43 

42 

41 

40 

39 

K 

36 

35       33 

32 

31 

29 

28 

26 

24 

23 

60 

0.517 

45 

44 

43 

41 

40 

M 

38 

36       35 

33 

32 

31 

29 

28 

26 

25 

61 

.536 

46 

45 

44 

43 

42 

411 

39 

38       36 

35 

34 

32 

31 

29 

28 

26 

62 

.555 

47 

46 

45 

44 

43 

42 

41 

39       38 

37 

35 

34 

32 

31 

30 

28 

63 

.575 

4x 

47 

46 

4.-> 

44 

43 

42 

41 

40 

38 

37 

36 

34 

32 

31 

30 

64 

.595 

50 

49 

48 

46 

45 

44 

43 

42 

41 

40 

38 

37 

36 

34 

33 

31 

65 

0.616 

51 

50 

49 

IN 

47 

46 

45 

43 

42 

41 

40 

38 

37 

36 

34 

33 

66 

.638 

:,-2 

51 

50 

49 

48 

47 

40 

45 

44 

42 

41 

40 

39 

37 

36 

35 

67 

.661 

53 

52 

51 

50 

49 

IS 

47 

46 

45 

44 

43 

42 

40       39 

38 

36 

68 

.684 

54 

53 

53 

52 

51 

.-,() 

49 

47 

46 

45 

44 

43 

42 

41 

39 

38 

69 

.707 

56 

55 

54 

53 

52 

51 

50 

49 

48 

47 

46 

44 

4.3 

42 

41 

39 

70 

0.732 

57 

56 

55 

54 

53 

52 

51 

50 

49 

48 

47 

46 

45  . 

44 

42 

41 

71 

.757 

58 

57 

56 

55 

54 

53 

52 

51 

50 

49 

48       47 

46 

45 

44 

42 

72 

.783 

59 

58 

57 

56 

55 

55 

54 

53 

52 

51 

50 

49 

47 

46 

45 

44 

73 

.810 

M 

59 

58 

58 

M 

:,.-, 

54 

53 

52 

51 

10 

49 

4s 

47 

46 

74 

.838 

61 

60 

60 

59 

58 

:,7 

56 

55 

54 

53 

52 

51 

50 

49 

48 

47 

75 

0.866 

62 

62 

61 

60 

59 

M 

57 

56 

55 

54 

53 

53 

52 

50 

49 

48 

76 

.896 

64 

63 

62 

61 

60 

59 

58 

57 

57 

.-,.; 

55 

54 

53 

52 

51 

50 

77 

.926 

65 

t>4 

63 

62       61 

60 

60 

59 

58 

57 

56 

55 

54 

53 

52 

51 

78 

.957 

M 

65 

64 

63 

63 

•2 

61 

60 

59 

58 

57 

H 

55 

54 

53 

52 

79 

989 

67 

66 

65 

64 

M 

62 

61 

60 

59 

58 

58 

57 

56 

55 

54 

80 

1.022 

68 

67 

66 

66 

6.-> 

.64 

63 

62 

62 

61 

60 

59 

58 

57 

56 

55 

86 


MANUAL   OF   AEEOGRAPHY. 


TABLE  V. —  Temperature  of  dew-point  in  degrees  Fahrenheit. 
[Pressure  =  27.0  inches.] 


Air 

Depression  of  wet-bulb  thermometer  (t  —  t'). 

tomp. 

16.5 

17.0 

17.5 

18.0 

18.5 

19.0 

19.5  20.0 

20.5 

21.0 

21.5 

22.0  22.5 

23.0 

23.5 

24.0 

47 

-3,7 

48 

-20 

-42 

49 

-12 

-22 

-47 

50 

-  6 

-13 

-24 

-56 

51 

-  2 

-  7 

-14 

-26 

52 

+  2 

-  2 

-  8 

-16 

-29 

53 

6 

+  2 

-  3 

-  8 

-17 

-32 

54 

9 

6 

+  1 

-  3 

-  9 

-18 

-35 

55 

12 

9 

5 

+  1 

—  4 

-10 

-19 

-39 

56 

14 

12 

9 

5 

+  1 

-  4 

-11 

-20 

-44 

57 

16 

14 

11 

8 

5 

±  0 

-  5 

-12 

-22 

-49 

58 

19 

16 

14 

11 

8 

+  5 

±  0 

-  5 

-12 

-23 

-57 

59 

21 

19 

16 

14 

11 

8 

+  4 

±  0 

-  5 

-12 

-24 

60 

23 

21 

19 

16 

14 

11 

8 

+  4 

±  0 

-  6  -13 

-25 

61 

25 

23 

21 

19 

16 

14 

11 

8 

+  4 

±0-6 

-14 

-26 

62 

26 

25 

23 

21 

19 

16 

14 

11 

8 

+  4-1 

-  6 

-14 

-27 

63 

28 

27 

25 

23 

21 

19 

17 

14 

11 

8+4 

-  1 

-  6 

-14 

-28 

64 

30 

28 

27 

25 

23 

21 

19 

17 

14 

11    8 

+  4 

-  1 

-  6  j-15 

-29 

65 

31 

30 

28 

27 

25 

23 

21 

19 

17 

14    11 

8 

+  4 

-  1 

-  6 

-15 

66 

33 

32 

30 

29 

27 

25 

23 

21 

19 

17 

14 

11 

8 

+  4 

1 

-  6 

67 

35 

33 

32 

30 

29 

27 

25 

24 

22 

19 

17 

14 

11 

8 

+  4 

-  1 

68 

36 

35 

34 

32 

31 

29 

27 

26 

24 

22   20 

17 

14 

12 

8 

+  4 

69 

38 

37 

35 

34 

32 

31 

29 

27 

26 

24 

22 

20 

17   15 

12    9 

70 

40 

38 

37 

35 

34 

32 

31 

29 

28 

26 

24 

22 

20   18   15    12 

71 

41 

40 

39 

37 

36 

34 

33 

31 

30 

28 

26 

24 

22 

20 

18    15 

72 

43 

42 

40 

39 

38 

36 

34 

33 

31 

30 

28 

26 

24 

23 

20 

18 

73 

44 

43 

42 

40 

39 

38 

36 

35 

33 

32 

30 

28 

27 

25   23 

21 

74 

46 

45 

43 

42 

41 

39 

38 

37 

35 

34 

32 

30 

29 

27   25 

23 

75 

47 

46 

45 

44 

42 

41 

40 

38 

37 

35 

34 

32 

31 

29   27 

25 

76 

49 

48 

46 

45 

44 

43 

41 

40 

39 

37 

36 

34 

32 

31   29 

27 

77 

50 

49 

48 

47 

46 

44 

43 

42 

40 

39 

38 

36 

34 

33 

31 

30 

78 

51 

50 

49 

48 

47 

46 

45 

43 

42 

41 

39 

38 

36 

35 

33 

32 

79 

53 

52 

51 

50 

49 

47 

46 

45 

44 

42 

41 

40 

38 

37  i  35 

34 

j 

80 

54 

53 

52 

51 

50 

49 

48 

M 

45 

44 

43 

42 

40 

39  i  37 

36 

Depression  of  wet-bulb  thermometer  (t—  t'). 

t 

24.5 

25.0 

25.5 

26.0 

26.5 

27.0 

27.5 

28.0 

28.5 

29.0 

29.5  30.0 

30.  5  j  31.0 

31.5 

32.0 

65 

-29 

66 

-15 

-30 

67 

-  6 

-15 

-30 

4 

68 

±  0 

-  6 

-15 

-30 

69 

+  5 

±  o 

-  6 

-15 

-30 

70 

9 

+  5 

±  0 

+  6 

-14 

-30 

71 

12 

9 

+  5 

±  0 

-  6 

-14 

-29 

72 

15 

12 

9 

+  5 

±  0 

-  5 

-14 

-28 

73 

18 

16 

13 

10 

+  6 

+  1 

-  5 

-13 

-27 

74 

21 

19 

16 

13 

10 

6 

+  1 

-  4 

-12 

-26 

75 

23 

21 

19 

16 

13 

10 

6 

+  2 

—  4 

-12 

—25 

76 

26 

24 

22 

19 

16 

14 

11 

7 

+  2 

-  4 

-11  -24 

77 

28 

26 

24 

22 

20 

17 

14 

11 

7 

+  3 

—  3  —11 

—23 

78 

30 

28 

26 

24 

22 

20 

17 

14 

11 

8 

+  3-2 

-10  -21 

79 

32 

30 

28 

27 

25 

23 

20 

18 

15 

12 

8  +  4 

-  2 

-  9 

-20 

-54 

80 

34 

32 

31 

29 

27 

25 

23 

21 

18 

15 

12    9 

+  4 

-  1 

-  8 

-18 

MANUAL    OF   AEROGRAPH Y. 
CLOUD  CLASSIFICATION. 


87 


International  system  of  cloud  classification,  and  definitions  pro- 
pu-e<l  by  Hildebrandsson  and^  Abercromby  and  adopted  by  the 
International  Meteorological  Congress  in  1894  and  the  average  height 
of  the  various  typos  at  different  locations  and  during  different  times 

of  year  follow : 

Average  height  in  meters. 


Latitude. 

l'i  t>dam, 
1806-97,  52°  N. 

Blue  Hill, 
1890-91  and 
7.42*°. 

Toronto, 
:.  43.0°. 

Washmuton, 
1896-97,  39°. 

Allaha- 
bad 

(India), 
1896-97, 
25!°. 

Manila, 
1890-87, 

15°. 

Sum- 
mer. 

Win- 
ter. 

Sum- 
mer. 

Win- 
ter. 

Sum- 
mer. 

Win- 
ter. 

Sum- 
mer. 

Win- 
ter. 

<  'UTU«! 

Meter*. 

9,100 
8,100 
5.900 
3,300 
3.600 
2,200 
2,100 
1,400 
4,700 
(3,800) 
1  800 

8,100 
7,600 

:>,  400 

3,000 

1,400 
1,700 
1,000 
4,000 
2,100 
1,300 

Mtters. 
9,500 
10,100 
6,700 
6,300 
3,800 
1,200 
2,900 
1,800 
9,000 
1.600 
1,200 

Meters. 
8,000 
8,900 
(i,200 
4,600 
3,700 
1,000 
l,fOO 
1,500 

10,900 
8,900 
8,900 
4,200 
3,500 
2,000 

Mtt(f.-: 
10,000 
8,500 
8,300 
4,200 
2,500 
1,500 

Meters. 

10,  400 
10,  <>00 
8,800 
5,800 
5.000 
2,900 
3,100 

Mtters. 
9,500 
9,500 
7,400 
4,800 
3.800 
2,400 
2,900 

Meters. 
12,400 
13,200 
11,400 

Meltr.'. 
10,900 
11,400 
6,000 
4,300 
5,300 
2,000 

Cirm-str  Uus  

Cirro-cumulus  
Alt.  ^iratus  
Alt  o-ci  i  in  u  1  us  

5,800 
3,500 

Sir  Uo-fumulus 

Cumulus  (S.)  
Cumulus  (B.)  
Cumulo-nimbus  (S.) 
Cumulo-nimbus  (B.) 
Nimbus 

1,700 

1,300 

1,200 
5  000 

1  200 
3  700 

1,400 

1,700 

1  000 

2,100 
2  000 

700 

l'900 

1  800 

1  ,  500 

S.  and  B.  refer  to  the  summits  and  bases,  respectively,  of  the  clouds  specified. 

Clouds — 1.  Cirrus  (Ci.). — Isolated  feathery  clouds  of  fine  fibrous 
texture  generally  of  a  white  color,  frequently  arranged  in  bands 
which  spread  like  the  meridians  on  a  celestial  globe  over  a  part  of 
the  sky  and  converge  in  perspective  toward  one  or  two  opposite 
points  of  the  horizon.  In  the  formation  of  such  bands  Ci.  S.  and 
Ci.  Cu.  often  take  part. 

2 .  Cirro-stratus  (Ci.  S.) . — Fine  whitish  veil,  sometimes  quite  diffuse, 
giving  a  whitish  appearance  to  the  sky,  and  called  by  many  "  cirrus 
haze,"    sometimes    of   more    or   less    distinct   structure,    exhibiting 
tangled  fibers.     The  veil  often  produces  halos  around  the  sun  and 
moon. 

3.  Cirro-ninni/iix  (Ci.  Cu.). — Fleecy  cloud:    Small  white  balls  and 
wisps  without   shadows,    or  with   very  faint   shadows,   which   are 
arranged  in  groups  and  often  in  rows. 

4.  Alto-cumulus  (A.  Cu.). — Dense  fleecy  cloud:  Larger  whitish  or 
grayish  balls  with  shaded  portions  grouped  in  flocks  or  rows,  fre- 
quently so  close  together  that  their  edges  meet.     The  different  balls 
are  generally  large  and  more  compact  (passing  into  S.  Cu.)  towrard 
the  center  of  the  group,  and  more  delicate  and  wispy  (passing  into 
Ci.  Cu.)  on  its  edges.     They  are  very  frequently  arranged  in  lines  in 
one  or  two  directions.     The   term   " cumulus  cirrus"   is  given   up 
because  it  causes  confusion. 

5.  Alto-stratus  (A.  S.). — Thick  veil  of  a  gray  or  bluish  color  exhibit- 
ing in  the  vicinity  of  the  sun  and  moon  a  brighter  portion,  which, 


88  MANUAL   OF    AEROGRAPH  Y. 

without  causing  halos,  may  produce  coronas.  This  form  shows 
gradual  transitions  to  cirro-stratus,  but  according  to  the  measurements 
made  at  Upsala  it  has  only  half  the  altitude.  The  term  "  stratus- 
cirrus''  is  abandoned  because  it  gives  rise  to  confusion. 

6.  Strato-cumulus   (S.    fV). — Large  balls  or  rolls  of  dark  cloud 
which  frequently  cover  the  whole  sky,   especially  in  wrinter,   and 
give  it  at  times  an  undulated  appearance.     The  stratum  of  strato- 
cumulus  is  not  usually  very  thick  and  blue  sky  often  appears  in  the 
breaks  through  it.     Between  this  form  and  the  alto-cumulus  all  possible 
gradations  are  found.     It  is  distinguished  from  nimbus  by  the  ball- 
like  or  rolled  form,  and  because  it  does  not  tend  to  bring  rain. 

7.  Nimbus   (N.). — Rain  clouds:  Dense  masses  of  dark,  formless 
clouds  with  ragged  edges,  from  which  generally  continuous  rain  or 
snow  is  falling.     Through  the  breaks  in  these  clouds  is  almost  always 
S3en  a  high  sheet  of  cirro-stratus  or  alto-stratus.     If  the  mass  of 
nimbus  is  torn  up  into  small  patches,  or  if  low  fragments  of  cloud 
are  floating  much  below  a  great  nimbus,  they  may  be  called  "fracto- 
nimbus, "  the  "scud"  of  the  sailors. 

8.  Cumulus  (Cu.}. — Wool  pack  clouds.     Thick  clouds  whose  sum- 
mits are  domes  with  protuberances  but  whose  bases  are  flat.     These 
clouds  appear  to  form  in  a  diurnal  ascensional  movement,  which  is 
almost  always  apparent.     When  the  cloud  is  opposite  the  sun  the 
surfaces  which  are  usually  seen  by  the  observer  are  more  brilliant 
than  the  edges  of  the  protuberances.     When  the  illumination  comes 
from  the  side  this  cloud  shows  a  strong  actual  shadow;  on  the  sunny 
side  of  the  sky,  however,  it  appears  dark  with  bright  edges.     The 
true  cumulus  shows  a  sharp  border  above  and  below.     It  is  often 
torn  by  strong  winds,   and  the  detached  parts  present  continual 
changes  ("fracto-cumulus"). 

9.  Cumulo-nimbus    (Cu.    N.). — Thundercloud    or    shower    cloud. 
Heavy  masses  of  cloud,  rising  like  mountains,   towers,   or  anvils, 
generally  surrounded  at  the  top  by  a  veil  or  screen  of  fibrous  texture 
("false  cirrus")  and  below  by  nimbus-like  masses  of  cloud.     From 
their  base  generally  fall  local  showers  of  rain  or  snow  and  sometimes 
hail  or  sleet.     The  upper  edges  are  either  of  compact  cumulus-like 
outline   and  form  massive  summits,   surrounded  by  delicate  false 
cirrus,  or  the  edges  themselves  are  drawn  out  into  cirrus-like  filaments. 
This  last  form  is  most  common  in  spring  showers.     The  front  of 
thunderstorm  clouds  of  wide  extent  sometimes  shows  a  great  arch 
stretching  across  a  portion  of  the  sky,  which  is  uniformly  lighter  in 
color. 

10.  Stratus  (S.). — Lifted  fog  in  a  horizontal  stratum.     When  this 
stratum  is  torn  by  the  wind  or  by  mountain  summits  into  irregular 
fragments,  the  clouds  may  be  called  "fracto-stratus." 


MANUAL  OF  AEROGRAPH Y.  89 

The  committee  also  adopted  the  following  instructions  for  recording 
clouds : 

1  The  kind  of  cloud  designated  by  jthe  international  letters  of  the  cloud  name, 
which  may  be  more  exactly  denned  by  giving  the  number  of  the  picture  in  the  atlas 
most  nearly  representing  the  observed  form. 

'2.  The  direction  from  nhich  (he  clouds  conie. — If  the  observer  remains  completely  at 
rest  during  a  few  seconds,  the  motion  of  the  clouds  may  easily  be  studied  by  noting 
their  relative  position  to  a  steeple  or  other  tall  object,  such  as  a  mast,  in  an  open  space. 

If  the  motion  of  the  cloud  is  very  slow,  for  such  an  observation  one's  head  must 
be  supported.  Clouds  should  be  observed  in  this  way  only  near  the  zenith,  for  if 
they  are  too  far  away  from  it  the  perspective  may  cause  errors.  In  this  case  nepho- 
scopes  should  be  used  and  the  rules  followed  which  apply  to  the  particular  instrument 
employed. 

3.  Radiant  point  of  the  upper  clouds. — These  clouds  often  appear  in  the  form  of  fine 
parallel  bands,  which  by  an  effect  of  perspective  seem  to  come  from  one  point  of  the 
horizon.     The  radiant  point  is  that  point  where  these  bands  or  their  direction  pro- 
longed meet  the  horizon.    The  position  of  this  point  on  the  horizon  should  be  re- 
corded in  the  same  way  as  the  wind  direction,  N.,  NNE.,  and  so  on. 

4.  I'lidulatory  clouds. — If  often  happens  that  the  clouds  show  regular  parallel  and 
equidistant  striM-.  like  the  waves  on  the  surface  of  water.     This  is  the  case  for  the 
greater  part  of  the  cirro-cumulus,  strato-cumulus  (roll-cumulus),  and  similar  forms. 
It  is  important  to  note  the  direction  of  these  stria\     When  there  are  apparently  two 
distinct  systems,  as  are  .to  be  seen  in  clouds  separated  into  balls  by  streaks  in  two 
directions,  the  directions  of  the  two  systems  should  be  noted.     As  far  as  possible 
observations  should  be  made  on  streaks  near  the  zenith  to  avoid  effects  of  perspective. 

5.  Density  and  position  of  cirrus  banks. — The  upper  clouds  frequently  take  the  form 
of  a  tangled  web,  or  of  a  more  or  less  dense  veil,  which,  rising  above  the  horizon,  re- 
sembles a  thin  white  or  grayish  bank.     As  this  cloud  form  has  an  intimate  relation  to 
barometric  depressions,  it  is  important  to  note : 

(a)  the  density;  0,  meaning  very  thin  and  irregular;  1,  meaning  thin  but  regular; 
2,  meaning  rather  dense;  3,  meaning  dense;  4,  meaning  very  dense  and  of  dark  color; 
(6)  the  direction  in  which  the  veil  or  bank  appears  densest. 
Remarks. — All  interesting  details  should  be  noted,  for  example: 

1.  On  summer  days  all  low  clouds  generally  assume  particular  forms  more  or  less 
resembling  cumulus.     In  this  case  there  should  be  put  under  "Remarks,"  "Stratus 
or  nimbus  cumuliformis." 

2.  It  sometimes  happens  that  a  cumulus  has  a  mammillated  lower  surface.     This 
appearance  should  be  described  by  the  name  of  "mammato-cumulus." 

3.  It  should  always  be  noted  whether  the  clouds  appear  stationary  or  whether  they 
have  a  very  great  velocity. 

Clayton,  in  the  Discussion  of  the  Cloud  Observations,  says  that— 

By  following  the  changes  in  nomenclature  since  Howard,  it  seems  clear  that  there 
has  been  a  gradual  evolution,  during  which  differences  and  distinctions  not  recognized 
by  Howard  have  been  established,  and  errors  due  to  perspective,  as  in  the  case  of  the 
cumulo-strattis.  have  been  corrected.  Thus  distinctions  between  high  and  low  cirro- 
stratus  and  between  high  and  low  cirro-cumulus  have  been  established,  and  the  lower 
forms  called  alto-stratus  and  alto-cumulus,  respectively.  The  stratus  has  been  sepa- 
rated into  fog  and  low  sheet  clouds,  and  two  distinct  forms  of  rain  cloud  are  recog- 
nized. These  distinctions  have  been  a  gradual  growth,  and  Abercromby  says:  "At 
Prof.  Hildebrandsson's  suggestion  we  examined  the  nomenclature  used  by  different 
officers,  and  arranged  the  names  systematically;  and  we  found  that  the  differences  did 
not  seem  irreconcilable.  Eventually,  we  agreed  that  10  terms,  all  compounded  of 


90  MANUAL  OF   AEROGRAPH Y. 

Howard's  four  fundamental  types — cirrus,  stratus,  cumulus,  nimbus — would  fulb 
meet  the  requirements  of  practical  meterorology,  with  the  least  disturbance  of  existing 
systems."  l  Hildebrandsson  further  says  that  the  10  cloud  forms  described  were 
already  recognized  in  the  nomenclature  used  in  Portugal.  Hence  the-  international 
cloud  nomenclature  adopted  at  Munich  represents  the  greatest  progress  in  cloud 
nomenclature  which  observers  are  yet  ready  to  accept  for  general  use .  and  no  official 
bureau  should  hesitate  to  accept  it  for  fear  that  the  system  is  merely  temporary  and 
will  soon  be  changed.  Progressive  development  will  undoubtedly  continue,  but 
changes  of  names  in  general  use  will,  in  all  probability,  be  slow.  A  more  detailed 
nomenclature  is,  however,  needed  for  the  use  of  specialists. 

Distribution  of  the  various  types  of  clouds.  — Bigelow  shows 
graphically  the  distribution  of  the  several  clouds.  Under  the  name 
of  each  type  he  gives  the  mean  height  in  meters  for  the  year  and  the 
number  of  observations.  There  is  also  plotted  for  the  several  types 
the  curve  of  frequency,  with  heights  as  ordinates  and  the  number  of 
observations  at  the  respective  heights  as  abscissas.  The  curves 
follow  the  mean  line  of  the  plotted  points  very  closely.  Under  the 
assumption  that  the  observed  frequency  corresponds  with  the  actual 
frequency  of  cirrus  formation  at  the  given  height,  a  discussion  of 
these  curves  would  give  a  good  explanation  of  the  physical  processes 
operative  in  cloud  formation  for  the  whole  year. 

There  is  a  wide  range  hi  the  heights  of  certain  clouds.  The  mean 
height  of  the  low  clouds  is  probably  2,000  meters.  The  three  low 
cloud  strata  are  shallow  (not  exceeding  3,000  meters  hi  depth), 
except  the  cumulo-nimbus,  or  thunder  head,  which  may  develop  a 
height  of  13,000  meters.  All  the  clouds  except  stratus  and  cumulo- 
nimbus show  a  tendency  to  three  maxima  of  height  and  thickness, 
one  hi  midsummer  and  the  other  two  in  February  and  November. 
The  minima  occur  in  March  and  September.  A  similar  relation  is 
found  to  exist  in  the  isothermal  limits. 

Cirrus  bands  have  been  explained  as  due  to  differences  in  velocity 
or  in  direction  of  contiguous  upper-air  currents.  These  currents 
nearly  always  move  from  west  to  east,  and  the  higher  part  of  the 
current  may  move  more  rapidly  than  the  lower.  Thus  the  upper 
part  of  any  cloud  formation  might  move  in  advance  of  the  base, 
causing  a  band  or  bar  extending  from  west  to  east. 

WAVE  MOTIONS  IN  THE  AIR  SHOWN  BY  CLOUD  UNDULATIONS. 

Cloud  billows,  or  undulations,  have  a  different  origin  from  cirrus 
bands,  though  it  is  not  always  easy  to  distinguish  between  the  two. 
According  to  Clayton,  bands  are  usually  isolated  or  widely  separated 
and  are  of  unequal  length,  while  undulations  are  close,  parallel  rows 
or  striations  of  nearly  equal  length.  The  undulations  were  com- 
paratively little  observed  until  Helmholtz  called  attention  to  them 
as  illustrating  wave  motions  in  the  air,  of  the  same  nature  as  ocean 

1  Quarterly  Journal  of  the  Royal  Meterological  Society,  Apr.,  1887,  p.  155. 


MANTAL   OF   AEEOGRAPHY.  91 

waves.  These  undulations  are  visible  in  clouds  at  all  altitudes. 
They  are  illustrated  in  the  strato-cumulus  by  long  parallel  rows, 
which  are  parallel  in  fact  as  well,  as  in  appearance,  and  lie  in  approxi- 
mately the  same  direction  in  all  parts  of  the  sky.  This  can  be  seen 
by  laying  a  ruler  across  the  center  of  the  mirror  of  the  nephoscope 
parallel  with  the  undulations.  Abercromby  had  a  different  opinion, 
but  he  clearly  made  no  critical  observations  in  this  way.  The 
undulations  are  illustrated  in  the  cumulus  level  by  long  parallel 
lines  formed  by  individual  cumuli  like  a  file  of  soldiers,  and  the  lines 
appear  to  converge  toward  the  horizon  as  the  effect  of  perspective. 
The  undulations  appear  to  be  most  frequent  in  the  alto-cumulus 
level,  and  are  easily  distinguished  by  the  parallel  rolls  in  the  alto- 
cumulus and  by  striations  in  the  alto-stratus,  like  the  furrows  in 
plowed  ground.  In  the  cirrus  level  they  are  usually  distinguished 
as  short,  parallel  threads  or  as  small  bands  forming  one  broad  band 
at  right  angles  to  their  length.  Sometimes  they  seem  like  furrows 
in  the  cirro-stratus. 

The  direction  of  length  of  the  undulations  is  decidedly  most  fre- 
quent from  north  to  south,  which  is  at  right  angles  to  the  most 
frequent  direction  of  cirrus  bands.  It  leads  at  once  to  the  inference 
that  cloud  undulation  is  the  phenomenon  to  which  the  popular  name 
of  l< polar  bands"  was  applied  in  Europe,  but  not  to  cirrus  bands,  as 
many  meteorologists  have  supposed  and  have  thus  been  led  to  intro- 
duce a  wrong  usage  of  the  term.  The  tendency  for  the  undulations 
to  lie  at  right  angles  to  their  motion  is  very  distinct,  and  is  in  contrast 
with  the  cirrus  bands. 

Since  the  crest  of  a  wave  usually  lies  at  right  angles  to  the  wind 
which  originates  and  drives  it  forward,  it  follows  that  the  results  of 
observation  agree  very  well  with  Helmholtz's  explanation  of  the 
cloud  undulations;  namely,  that  the  clouds  are  the  visible  crests  of 
real  atmospheric  waves  formed  between  currents  of  air  of  a  different 
density  and  having  a  different  velocity  or  direction.  The  undula- 
tions would  probably  always  lie  at  right  angles  to  the  upper  current 
were  it  not  that  the  lower  current  is  also  in  motion,  and  that  the 
observed  cloud  direction  is  a  compound  of  the  two. 

The  value  of  clouds  in  forecasting  weather  changes. — The 
cloud  is  not,  as  might  be  expected  at  first  thought,  a  good  exponent 
of  air  motion;  and  as  yet  cloud  maps  have  not  been  used  advantage- 
ously by  professional  forecasters  except  in  connection  with  storms 
of  the  West  Indian  hurricane  type,  or  the  typhoon,  or  baguio,  of  the 
China  Sea.  Thus  Father  Vines,  at  Havana,  showed  how  certain 
types  of  cirrus  accompanied  or  rather  preceded  storms  of  great 
violence  but  small  diameter,  while  a  different  type  was  found  to 
accompany  storms  of  large  diameter  and  moderate  violence.  Like- 
wise at  Manila,  Zi-ka-wei,  and  other  observatories  on  the  Asiatic 


92  MANUAL  OF   AEEOGEAPHY. 

coast,  the  appearance  and  motion  of  the  upper  clouds  have  been 
carefully  studied  for  forecasting  purposes. 

In  genera],  cirrus  clouds,  except  of  a  certain  type,  do  not  positively 
indicate  coming  rain,  being,  in  fact,  somewhat  less  frequently  fol- 
lowed by  rain  than  the  average  probability  of  rain.  But  they  are 
closely  controlled  in  their  movements  by  temperature  gradients,  and 
they  may  serve  an  isolated  observer  as  a  guide  to  coming  changes  of 
temperature.  In  general,  slowly  moving  cirrus  clouds  indicate 
slight  changes  in  temperature,  and,  except  when  moving  from  a 
direction  between  south  and  west,  they  indicate,  as  a  rule,  slowly 
rising  temperature  during  the  succeeding  12  and  24  hours.  Rapidly 
moving  cirrus  indicate  the  probability  of  decided  changes  of  tem- 
perature, and,  from  any  direction,  a  probability  of  a  fall  of  tempera- 
ture by  the  end  of  24  hours.  The  probability  of  a  fall,  however,  and 
the  amount  of  fall,  are  much  greater  and  earlier  when  the  cirrus  are 
observed  to  be  moving  rapidly  from  a  direction  between  south  and 
west.  When  cirrus  are  observed  to  be  moving  from  the  southwest, 
there  is  a  strong  probability  of  a  fall  of  temperature  during  the  suc- 
ceeding 24  hours.  This  probably  rises  to  83  per  cent  in  winter  and 
is  over  70  per  cent  at  all  times  of  the  year  for  cirrus  moving  rapidly 
from  the  southwest.  When  cirrus  are  observed  to  be  moving  from 
the  northwest,  the  probability  is  that  there  will  be  a  rise  of  tempera- 
ture during  the  succeeding  12  hours.  The  probability  is  64  per  cent 
for  winter  and  76  per  cent  during  the  entire  year  for  cirrus  moving 
rapidly  from  the  northwest. 

With  the  appearance  of  cirro-stratus  there  is  a  probability  of  rain 
during  the  succeeding  24  hours  of  about  80  per  cent.  This  proba- 
bility increases  to  nearly  90  per  cent  with  the  appearance  of  alto- 
stratus,  which  is  as  great  as  can  usually  "be  derived  from  a  knowledge 
of  the  conditions  prevailing  over  the  country  as  given  on  a  weather 
map.  Cirro-cumulus  are  most  frequently  followed  by  fair  weather, 
while  alto-cumulus  indicate  a  probability  of  rain. 

There  are  two  directions  in  which  observations  of  the  direction 
and  of  the  relative  velocity  of  upper  clouds  might  be  of  use.  The 
rapid  movement  of  cirrus  from  the  west  or  the  southwest  along  the 
northern  boundary  of  the  United  States  will  no  doubt  indicate  the 
approach  of  a  cold  wave  before  its  approach  is  indicated  by  the 
weather  map,  and  will  thus  enable  northwestern  stations  to  be  warned. 
The  movement  of  cirrus  from  the  south,  observed  at  any  of  the 
Atlantic  coast  stations,  with  a  dense  bank  of  clouds  to  the  south  of 
the  observer,  would  strongly  indicate  a  severe  storm  off  the  coast 
and  might  enable  the  observer  to  determine  the  position  of  its  center. 
This  conclusion  is  derived  from  the  individual  observations  at  Blue 
Hill  and  from  the  fact  of  circulation  of  air  in  cyclones  with  deep 


MANUAL   OF   AEROGRAPH  V.  93 

barometric  depressions.  The  prevalence  of  rapidly  moving  cirrus 
over  a  wide  area  indicates  rapid  storm  movement  and  rapid  and 
marked  changes  of  weather  and  temperature.  Slowly  moving  cirrus 
indicate  sluggish  storm  movements  and  slight  changes  of  tempera- 
ture and  are  the  usual  accompaniment  of  droughts. 

The  direction  of  cirrus  movement  prevailing  in  advance  of  and 
around  the  storm  center  must,  in  the  majority  of  cases,  furnish  a 
clew  to  the  future  movements  of  the  storm,  since  it  is  found  that  the 
storm  tends  to  move  in  the  mean  direction  of  the  cirrus  found  for 
the  storm  as  a  whole. 

A.  McA. 


CHAPTER  IX, 


RADIATION. 


95 


CHAPTER  IX. 
RADIATION. 

The  source  of  radiant  energy. — The  earth  is,  relative!;,  speak- 
ing, an  insignificant  unit  in  the  solar  system.  Furthermore,  the 
solar  system  itself  is  an  insignificant  unit  in  the  stellar  universe. 
Astronomers  tell  us  that  the  solar  system  is  moving  rapidly  toward 
the  constellation  Hercules;  but  no  appreciable  effect  upon  the  earth's 
atmosphere  is  known  to  result,  nor  is  the  amount  of  energy  received 
from  the  stars  sufficient  to  produce  any  observable  effect.  Efforts 
have  been  and  are  being  made  to  measure  the  scattered  radiation  of 
•the  sky;  but  as  yet  there  are  no  positive  results.  With  the  radiant 
energy  of  the  sun,  however,  it  is  dmerent;  and  here  we  have  to  deal 
with  a  prime  mover.  The  mean  distance  of  the  sun  from  the  earth 
is  140,500,000  kilometers,  or  92,900,000  miles.  The  solar  parallax  is 
8.796  seconds  and  the  sun's  diameter  1,392,000  kilometers,  or  865,000 
miles.  The  velocity  of  light  is  299,870  kilometers  per  second  (186.300 
miles),  and  the  time  required  to  traverse  the  mean  radius  of  the 
earth's  orbit  is  498.8  seconds.  The  visible  spectrum  comprises  light 
waves  ranging  in  wave  length  from  0.7  ju  (0.0007  mm.),  the  red  end, 
to  0.4  ju  (0.0004  mm..),  the  violet  end.  At  only  a  few  aerological 
observatories  are  records  of  the  intensity  of  solar  radiation  main- 
tained. Perhaps  one  of  the  most  serviceable  records  is  that  made 
at  Davos,  in  the  Swiss  Alps,  where  continuous  records  have  been 
obtained  by  C.  Dorno.  At  this  mountain  station  a  continuous  photo- 
graphic record  of  the  length  of  the  ultra-violet  spectrum  (that  is, 
the  value  of  the  shortest  waves  which  penetrate  the  atmosphere) 
shows  that  the  winter  sun  has  great  heating  effect,  but  apparently 
does  not  attain  a  maximum  in  the  other  end  of  the  spectrum.  The 
spring  sun  has  the  greatest  heat,  with  somewhat  greater  amount  of 
ultra-violet  radiation;  the  summer  sun,  much  heat  and  strongest 
ultra-violet;  and  the  autumn  sun,  much  heat  and  much  ultra-violet. 
Gockel  thinks  that  herein  lies  the  explanation  of  the  " glacier  burn," 
that  is,  an  intense  ultra-violet  radiation.  One  point  of  interest  is 
that  the  ultra-violet  radiation  undergoes  more  variation  than  the  heat : 
and  varies  greatly  with  the  season,  so  that  a  single  day  in  summer 
may  equal  a  winter  month's  total. 

.  The  intensity  of  solar  radiation  is  measured  by  the  heat  produced 
when  a  given  surface  exposed  at  right  angles  to  the  beam  entirely 
absorbs  the  radiant  energy.  The  mean  value  of  the  so-called  "solar 
constant  of  radiation"  has  been  fixed  by  Abbot  at  1.932  calories  per 
square  centimeter  per  minute.  This  value  diners  materially  from 
50821—18 7  97 


98  MANUAL   OF  AEROGEAPHY. 

former  values,  especially  the  generally  accepted  value  of  3  calories 
as  given  in  many  textbooks.  If  the  solar  constant  were  indeed  con- 
stant, the  earth  would  receive  in  a  year  something  like  one  million 
million  million  million  calories.  In  popular  terms  this  would  be- 
sufficient  heat  to  melt  a  layer  of  ice  33  meters  thick  over  the  entire 
surface  of  the  earth  annually,  or  to  evaporate  1.66X1033  kilograms 
of  water,  provided  there  were  no  atmosphere,  no  absorption,  and  no 
reflection. 

Variation  in  sunshine. — At  any  given  point  there  must,  of 
course,  be  variation  as  the  sun  changes  longitude.  Thus  on  Jan- 
uary 1,  when  the  longitude  is  1°,  the  ratio  is  1.03;  on  March  1, 
longitude  59°,  1.02;  on  July  1,  longitude  179°,  0.96;  on  September  1, 
longitude  240°,  0.98;  and  on  December  1,  longitude  330°,  1.03. 
Thus  in  winter  the  value  is  larger  than  in  summer.  The  duration 
of  sunshine  can  be  determined  for  any  given  latitude  from  the  hour 
angle  converted  into  mean  solar  time  and  then  multiplied  by  2. 
Considering  northerly  declination  positive,  and  southerly  declination 
negative,  we  have  for  example  in  latitude  42°  N.  the  following  values: 

Duration  of  sunshine. 


Declination  of  the  sun. 

Length  of  day. 

Declination  of  the  sun. 

Length  of  day. 

Hours. 

Minutes. 

Hours. 

Minutes. 

—  23°  27' 

9 
9 
10 
10 
11 
12 

7 
37 
18 
56 
33 
9 

5° 

12 
13 
14 
14 
15 

45 
22 
1 
43 
14 

—20°.        . 

10°  

15° 

—  15° 

-10°                        

20°  

23°  27' 

0° 

The  greatest  possible  duration  for  other  latitudes  is: 


Latitude. 

0° 

20° 

40° 

60° 

66° 

90° 

Maximum  insolation                         

H.  m. 
12    7 

H.  m. 

13    20 

H.m. 
15    1 

H.m. 
18    52 

H. 

24 

6  months 

If  the  unit  of  insolation  be  the  amount  received  in  a  day  at  the 
Equator  on  March  21,  then  for  given  latitudes  values  will  vary  in  the 
following  ratios: 


Latitude. 


Dates. 

0° 

20° 

40° 

60° 

North 
Pole. 

South 
Pole. 

March  21 

1.00 

0.93 

0.76 

0.  50 

June  21 

.98 

1.04 

1.10 

1.09 

1.20 

Sept.  23  

.88 

.94 

.70 

.30 

Dec.  21.. 

.94 

.68 

.35 

1.28 

MANUAL   OF  AEROGRAPSV.  , 

The  orbit  which  the  sun  appears  to  make  around  the  earth,  but 
which  in  reality  is  made  by  the  earth  around  the  sun,  is  not  a  circle 
but  an  ellipse  inclined  to  the  plane  of  the  equator.  The  speed  of 
the  earth  is  not  constant;  and  instead  of  traveling  equal  distances 
in  equal  times,  the  distance  traveled  is  such  as  to  make  the  areas 
swept  over  by  the  line  joining  earth  and  sun  equal  in  equal  times. 
So  when  the  sun  is  nearest,  the  earth  travels  fastest.  As  we  have 
said,  the  sun  appears  to  travel  in  a  plane  which  makes  an  angle  of 
23°  with  the  plane  of  the  equator. 

There  may  be  other  causes  of  variation  in  the  intensity  of  solar 
radiation — changes  which  may  be  of  solar  origin  and  not  periodic. 
Thus  the  monthly  mean  values  of  the  solar  constant  from  1905  to 
1912  have  been  compared  with  the  so-called  "Wolff  sunspot  numbers" 
for  the  same  months,  and  it  seems  likely  that  increased  values  of 
the  solar  constant  attend  increased  sunspot  numbers.  In  the  report 
of  the  Astrophysical  Observatory  for  1913  it  is  stated  that  there  is 
an  increase  of  radiation,  at  the  earth's  mean  distance  from  the  sun, 
of  0.07  calorie  per  square  centimeter  per  minute  with  an  increased 
spottedness  ot  the  sun,  represented  by  100  Wolff  sunspot  numbers. 

Simultaneous  observations  at  Mount  Wilson  and  at  Bassour, 
Algeria,  indicate  that  fluctuations  in  solar-constant  values  found  in 
California  in  earlier  years  may  now  be  explained  not  as  local  phenom- 
ena but  as  due  to  causes  outside  of  the  earth;  and  thus  we  may 
conclude  that  the  sun  is  a  variable  star,  having  not  only  a  periodicity 
connected  with  the  periodicity  of  sunspots,  but  also  an  irregular, 
nonperiodic  variation,  sometimes  running  its  course  in  a  week  or 
10  days,  again  in  longer  periods,  and  ranging  over  irregular  fluctu- 
ations of  from  2  to  10  per  cent  of  the  total.  It  has  also  been  shown 
by  Abbot,  Fowle,  Kimball,  and  others  that  great  volcanic  eruptions 
materially  decrease  the  apparent  solar  radiation,  or  rather  that 
atmospheric  transmissibility  undergoes  marked  changes  with  conse- 
quent diminution  ot  temperature.  Marked  changes  occurred  in 
1884-1886  (probably  connected  with  Krakatau)  and  again  in  1903-4. 

Measurement  of  solar  radiation. — By  using  a  Callendar 
pyrheliometer  and  an  eclipsing  screen,  the  total  radiation  can  be 
obtained  in  two  components,  one  representing  direct  solar  radiation 
and  the  other  the  diffuse  sky  radiation.  The  total  radiation  per 
square  centimeter  of  horizontal  surface  with  the  clearest  sky  varies, 
according  to  Kimball,  for  the  particular  point  of  observation  (near 
Washington),  from  250  calories  a  day  (December  20)  to  765  calories 
(June  10).  In  general  the  radiation  received  on  clear  days  during 
the  first  half  of  the  year  exceeds  that  of  the  second  half  by  8  per 
cent,  probably  due  to  the  increased  water- vapor  content  of  the 
atmosphere  during  the  latter  period. 


iao: 


MAtf^TAL   OF   AEROGEAPHY. 


The  total  radiation  received  with  the  clearest  sky  in  midday  per 
square  centimeter  of  horizontal  surface  varies  from  45  calories  in 
December  to  90  in  June.  When  clouds  are  near  the  sun  but  do  not 
obscure  it,  the  momentary  maximum  rates  are  increased  by  about 
0.15  calorie. 

The  diffuse  sky  radiation  received  on  a  horizontal  surface  at  noon 
averages  about  25  per  cent  of  that  from  the  sun. 

Expressed  in  units  of  work,  1  calorie  per  minute  per  cm.2  repre- 
sents 697  watts  per  m.2;  90  calories  per  hour  (1 -J  per  minute)  represent 
1  kilowatt  per  m. 2 

The  radiation  received  on  a  square  meter  of  horizontal  surface  on 
a  clear  day  in  midsummer  is,  therefore,  equivalent  to  5  kilowatt 
hours. 

Some  recent  measurements  are:  American  University,  Washington, 
D.  C.,  on  December  24,  1914,  with  the  sun  at  zenith  distance  62.5°, 
an  intensity  of  1.48  calories  per  minute  per  square  centimeter;  and 
on  February  28,  1915,  with  the  sun  at  zenith  distance  57.5  °  the 
intensity  was  1.50  calories.  At  Santa  Fe,  N.  Mex.,  elevation  2,133 
meters,  and  in  an  arid  region,  a  maximum  of  1.64  calories  was 
recorded  with  a  zenith  distance  of  55°.  In  brief,  at  sea  level,  in 
summer  and  at  midday,  there  reaches  the  earth  each  second  0.0225 
calorie  per  square  centimeter;  and  of  this  0.0096  calorie  is  scattered 
or  absorbed  and  0.006  calorie  reradiated  from  the  atmosphere. 
The  amount  of  energy  varies  inversely  as  the  square  of  the  distance 
from  the  sun,  with  the  angle  of  incidence  of  the  rays,  and  according 
to  duration. 

It  is  possible  that  there  is  in  the  upper  atmosphere  a  layer  of 
cosmical  dust  which  is  strongly  radioactive.  Simpson  has  recently 
pointed  out  that  the  measurements  of  Vegard  and  Stormer  on  the 
aurora  indicate  true  radioactive  radiation  penetrating  the  atmosphere 
and  producing  the  same  results  as  if  the  atmosphere  were  being 
bombarded  from  the  outside  by  the  a  radiation,  which  is  now  being 
studied  in  so  many  physical  laboratories.  Experiments  on  ioniza- 
tion  made  in  balloons  in  1914  show  the  existence  of  a  strong  radiation. 
This  may  help  explain  the  nature  of  the  aurora.  The  average  height 
of  the  bottom  edge  of  the  aurora  as  determined  by  1 ,920  measurements 
in  Norway  is  108  kilometers,  and  no  aurora  lower  than  85  kilometers 
was  noticed.  It  would  seem  that  the  cosmic  rays  producing  the 
aurora  are  in  two  groups  with  different  penetrating  power.  The 
diffuse  arcs,  the  drapery,  and  more  intense  displays  seem  to  be  of 
the  same  physical  nature. 

A.  McA. 


CHAPTER  X. 


ATMOSPHERIC  ELECTRICITY. 


101 


CHAPTER  X. 


ATMOSPHERIC  ELECTRICITY. 

THE    ELECTRICITY   OF   THE    AIR   AND    EARTH. 

It  is  now  generally  accepted  that  the  normal  potential  gradient1 
over  the  whole  globe  is  positive;  that  is,  the  whole  surface  of  the 
earth,  except  regions  of  disturbance,  has  a  negative  charge  of  elec- 
tricity. This  charge  undergoes  daily  and  yearly  variations  and  is  not 
constant. 

For  years  it  was  thought  that  pure  air  was  an  insulator,  but  from 
experiments  carried  on  in  1900  and  1901  by  Elster  and  Geitel  in 
Germany,  and  Wilson  in  England,  it  was  found  that  pure  air  is  not 
an  insulator,  but  that  it  always  contains  a  number  of  ions  2  which 
give  to  it  the  power  of  conducting  electricity  in  a  manner  similar  to 
that  of  an  electrolyte. 

Negative  electricity  is  dissipated  more  rapidly  than  positive  in  the 
lower  atmosphere,  as  there  are  more  positive  than  negative  ions  in 
the  air  near  the  surface,  which  is  a  consequence  of  the  former  being 
attracted  downward  and  the  latter  being  repelled  upward  by  the 
negative  charges  on  the  earth. 

One  of  the  primary  laws  of  electricity  in  speaking  of  negative  and 
positive  charges  is  that  like  charges  repel  and  unlike  charges  attract. 

As  no  dissipation  would  be  possible  without  ions,  and  no  ions  could 
be  produced  without  the  action  of  some  ionizer  possessing  the  neces- 
sary energy,  it  is  believed  that  there  are  three  main  ionizers  at  work 
in  the  atmosphere,  namely,  an  X-ray -like  radiation,  radio-active 
matter  in  the  ground,  and  radio-active  emanation  acting  in  the 
atmosphere. 

Radio-active  properties  are  exhibited  to  some  extent  in  all  minerals. 
A  supply  of  electricity  is  given  to  the  air,  in  various  amounts,  by  some 
substance  present  in  rocks.  Many  radio-active  substances  not  only 

1  The  potential  gradient  means  the  difference  of  electrical  potential  between  two  points  1  meter  apart 
vertically,  the  ground  surrounding  them  being  supposed  to  be  a  level  horizontal  plane.  The  value  of 
this  gradient  when  measured  near  the  ground  depends  entirely  on  the  surface  of  the  earth,  a  potential 
gradient  of  100  volts  per  motor  representing  an  electrical  charge  of  1(H3  coulombs  per  square  centimeter  of 
surface. 

1  An  ion  is  any  minute  material  particle  which  carries  an  electrical  charge.  Generally  an  ion  is  an  atom 
or  a  molecule  of  atmospheric  gas  carrying  an  elemental  charge  of  elect  ricit  y .  Two  ions,  one  positive  and  one 
negative,  are  produced  by  the  breaking  up  of  a  neutral  molecule  into  two  charged  atoms  or  two  charged 
molecules  of  smaller  dimensions.  lonization  is  the  process  of  the  formation  of  ions  by  the  splitting  up  of 
neutral  molecules. 

(Fefinitions  quoted  from  George  C.  Simpson.    Quart.  Journ.  Roy.  Met.  Soc.,  Oct..  1905.) 

103 


104  MANUAL  OF   AEROGRAPH Y. 

give  off  electric  charges,  but  also  a  gaseous  material  called  "emana- 
tion," which  has  itself  this  same  property  of  manufacturing  ions.1 

Positive  electricity  flows  from  places  of  high  to  places  of  low  po- 
tential. This  being  the  case,  the  electricity  in  the  atmosphere  will 
naturally  move  in  accordance  with  this  law.  Thus  positive  charges 
will  flow  from  the  air  to  the  earth  and  negative  charges  in  the  oppo- 
site direction.  This  flow  constitutes  an  electric  current  which  is 
known  as  the  air-earth  current. 

THE    ELECTRICITY   OF    RAIN,    THUNDER   AND   LIGHTNING. 

It  has  been  found  that  rain  is  nearly  always  electrically  charged, 
about  80  per  cent  of  all  rain  carrying  positive  electricity.  From 
observations  carried  on  by  Dr.  G.  C.  Simpson,  of  the  Indian  meteoro- 
logical department,  it  was  found  that  light  rain  carried  the  greatest 
charges,  with  the  exception  of  the  thunder  rain,  which  was  always 
found  to  be  more  highly  charged  than  rain  unaccompanied  by  thunder. 

In  a  thunderstorm  the  heavy  rain  which  occurs  at  the  beginning  of 
a  storm  has  a  high  positive  charge,  and  the  steady  uniform  precipita- 
tion following  being  also  highly  charged,  but  with  negative  electricity. 

Thunderstorms  are  almost  always  accompanied  by  strong  ascending 
air  currents  which  tend  to  break  up  the  drops  falling  through  them. 
As  the  drops  break  up  they  become  positively  charged  while  the  sur- 
rounding air  particles  receive  a  negative  charge.  The  air  thus  charged 
is  carried  up  by  the  ascending  current  and  away  from  the  positively 
charged  drops  into  contact  with  cloud  particles,  which  thus  acquire 
a  considerable  negative  charge.  These  clouds  provide  the  negatively 
charged  rain  of  the  latter  part  of  the  storm. 

Drops  when  electrically  charged  combine  more  rapidly  than  when 
uncharged,  and  in  this  way  the  process  of  recombination  goes  on 
rapidly,  the  drops  quickly  increasing  in  size  until  they  again  break 
up  with  another  separation  of  electricity,  the  positive  remaining 
on  the  drops  and  the  negative  on  the  air  particles.  The  air,  which  is 
full  of  free  negative  ions,  has  been  carried  up,  leaving  the  positively 
charged  drops  behind. 

The  electrical  separation  within  a  thunderstorm  cloud  is  such  as  to 
place  a  heavily  charged  positive  layer  (the  lower  portion  of  the  cloud) 
between  the  earth  and  a  much  higher  heavily  charged  negative  layer 
(the  upper  portion  of  the  cloud.)2  Discharges  of  lightning  may  take 
place  from  the  intermediate  or  positively  charged  layer  to  either  the 
negative  portion  above  or  to  the  earth. 

The  uprushing  air,  by  its  sustaining  influence  and  turbulence, 
forms  at  times  a  practically  continuous  sheet  or  stream  of  water, 
heavily  charged  and  at  high  potential,  and  also  layers  and  streaks 

1  C.  D.  Stewart:  Atmospheric  Electricity,  Quart.  Journ.  Roy.  Met.  Soc.,  October,  1917. 
8  W.  J.  Humphreys  "The  Thunderstorm:  Its  Phenomena." 


MANUAL   OF   AEROGRAPH Y.  105 

of  highly  ionized  air.  Electrically  speaking,  heavily  charged  con- 
ducting sheets  and  rods,  either  of  coalesced  drops  or  of  ionized  air,  are 
formed  over  and  over  so  long  as  the  storm  lasts,  momentarily  placed 
here  and  there  within  the  mass  of  the  storm  cloud  which  is  positively 
charged.  This  makes  a  heavily  surface-charged  vertical  conductor  in  a 
strongly  volume-charged  horizontal  layer  or  region,  above  and  below 
which  there  are  steep  potential  gradients  to  negatively  charged 
parallel  surfaces. 

The  conductor  will  be  at  the  same  potential  throughout,  and 
therefore  the  maxima  of  potential  gradients  normal  to  it  will  be  at 
its  ends,  where  if  these  gradients  are  steep  enough,  and  the  longer  the 
conductor  the  steeper  the  gradients,  brush  discharges  will  take  place. 
The  brush  discharge  and  the  line  of  its  most  vigorous  ionization 
necessarily  will  be  directed  along  the  potential  gradient  or  toward 
the  surface  of  the  opposite  charge.  But  this  very  ionization  auto- 
matically increases  the  length  of  the  conductor,  and  as  the  length 
of  the  conductor  grows,  so,  too,  does  the  steepness  of  the  potential 
gradient  at  its  forward  or  terminal  end,  and  as  the  steepness  of  this 
gradient  grows  the  more  vigorous  the  discharge,  always  assuming  that 
there  is  an  abundant  electrical  supply.  Therefore,  an  electric  spark 
once  started  within  a  thunderstorm  cloud  has  a  good  chance  by 
making  its  own  conductor  as  it  goes,  of  growing  into  a  tremendous 
lightning  flash.  When  the  electrical  supply  is  small,  then  lightning 
will  be  feeble  and  soon  dissipated.1 

Lightning  may  be  divided  into  three  main  classes  according  to  the 
form  of  discharge:  Forked,  sheet,  or  globe  lightning. 

Forked  lightning  is  a  zigzag  line  of  fire  similar  to  the  discharge  of  a 
Leyden  jar.  The  electric  current  flows  along  the  path  of  least 
resistance.  The  path,  owing  to  the  variations  in  the  atmospheric 
structure,  is  very  irregular  and  the  consequence  is  seen  in  the  zigzag 
path.  The  duration  of  the  lightning  discharge  is  probably  much  less 
than  the  hundred-thousandth  part  of  a  second. 

Sheet  lightning  consists  of  flashes  of  light  which  illuminate  the 
clouds  and  which  are  often  not  accompanied  by  thunder.  They  may 
be  ordinary  flashes  of  forked  lightning  invisible  to  the  observer,  owing 
either  to  distance  or  to  their  passing  from  cloud  to  cloud  without 
reaching  the  earth. 

Globe  lightning  is  a  mysterious  phenomenon  stated  to  consist  of  a 
ball  of  fire  moving  slowly  through  the  air,  sometimes  accompanied  by 
a  violent  explosion. 

Under  normal  air  pressure  it  has  been  estimated  that  the  electro- 
motive force  necessary  to  produce  a  spark  a  mile  long  is  over  3,000,000 
volts.  When  the  discharge  is  from  cloud  to  earth  the  length  of  the 

i  W.  J.  Humphreys,  "The  Thunderstorm:  Its  Phenomena." 


106  MANUAL  OF   AEROGRAPH Y. 

path  is  seldom  more  than  1J  to  2  miles.  In  the  case  of  low-lying 
clouds  it  may  be  much  less,  especially  so  when  they  envelop  a 
mountain  peak.  On  the  other  hand,  when  the  discharge  is  from  cloud 
to  cloud,  the  path  is  generally  more  tortuous  and  its  total  length 
much  greater,  not  exceeding  20  kilometers. 

THE    AURORA    BOREALIS. 

The  aurora  borealis,  or  northern  lights,  usually  consist  of  a  whitish 
arc  of  light  or  quivering,  rapidly  moving  beams.  An  arc  aurora 
consists  of  a  luminous  segment  of  a  circle.  A  form  of  aurora  which 
is  probably  a  modification  of  the  arc  form  often  appears  as  a  band. 

The  aurora  appears  with  a  variety  of  colors,  but  the  main  part  is 
usually  whitish,  accompanied  in  some  of  the  brighter  forms  by  a 
yellowish  tinge.  In  forms  which  are  not  so  bright  it  appears  to  be  a 
silvery-white  color.  If  the  light  is  very  intense  a  red  tint  may  be 
seen  about  the  lower  edge  and  sometimes  a  green  shade  appears  in  the 
position  nearest  the  zenith. 

The  height  of  the  aurora  above  the  earth  may  vary  between  wide 
limits.  Estimates  made  at  various  times  place  the  lowest  at  a  height 
of  about  0.6  kilometer  and  the  highest  at  about  68  kilometers.  The 
average  height  seems  to  be  about  20  kilometers. 

An  auroral  display  usually  takes  place  in  the  early  evening  and  may 
last  for  a  few  hours.  The  maximum  number  of  auroras  occur  in  March 
and  October  and  the  belt  of  maximum  frequency  in  the  northern 
hemisphere  extends  from  about  65°  to  85°  north  latitude. 

Auroras  are  believed  to  be  due  to  electric  currents  in  the  atmos- 
phere. It  is  thought  that  cathode  rays  emanate  from  the  sun. 
These  rays  travel  in  straight  lines,  unless  deflected  by  a  magnet  (the 
earth)  and  cause  bright  phosphorescence  when  they  fall  upon  ice 
particles  in  the  upper  atmosphere. 

ST.  ELMO'S  FIRE. 

The  electrical  phenomenon  known  as  Corposants,  or  St.  Elmo's  fire, 
which  is  frequently  seen  from  projecting  points,  such  as  the  masts  of 
vessels  in  low-hanging  clouds,  appears  when  atmospheric  electricity 
of  low  intensity  induces  electricity  on  the  ship  or  other  object  that 
happens  to  be  under  its  influence.  This  induced  electricity  concen- 
trates at  the  extremities  of  structures  either  at  sea  or  on  shore,  and 
becomes  visible  as  a  luminous  brush  discharge. 

A.  S.  M. 


CHAPTER  XL 


OPTICAL  PHENOMENA. 


107 


CHAPTER  XL 


OPTICAL  PHENOMENA. 

There  are  a  large  number  of  optical  phenomena  which  are  of  interest 
for  two  reasons :  For  their  beauty  and  because  they  are  more  or  less 
closely  connected  with  the  weather.  They  are  important  for  both 
reasons  and  are  worthy  of  careful  observation. 

The  phenomena  are  due  to  various  causes  and  take  different  forms, 
so  for  convenience  they  may  be  grouped  under  three  heads : 

(1)  Phenomena  which  are  due  to  the  gases  of  the  atmosphere  them- 
selves (refraction,  twinkling,  mirage,  and  looming);  (2)  those  due  to 
the  particles  sometimes  present  in  the  atmosphere  (halos,  rainbows, 
and  cloud  shadows) ;  (3)  those  due  to  the  small  particles  always  present 
in  the  atmosphere  (coloration  of  the  sky,  colors  of  sunset  and  sunrise, 
and  twilight).1 

Refraction. — According  to  a  law  in  optics  when  a  ray  of  light 
passes  from  a  medium  of  one  density  into  that  of  another,  it  is  bent 
from  its  course,  being  bent  toward  the  normal  to  the  bounding  surface 
when  passing  from  a  rarer  to  a  denser  medium,  and  vice  versa.  There- 
fore when  a  ray  of  light  enters  the  earth's  atmosphere  from  space  and 
passes  through  air  layers  of  steadily  increasing  density  it  must  be 
continuously  bent  toward  the  normal. 

Refraction  thus  has  the  effect  of  raising  an  object  or  increasing  its 
altitude  above  the  horizon.  At  the  zenith,  or  point  directly  overhead, 
the  amount  of  refraction  is  zero,  but  toward  the  horizon  it  steadily 
increases,  where  it  has  a  maximum  value  of  a  little  more  than  half 
a  degree.  As  the  amount  of  refraction  depends  upon  the  density  of 
the  air  it  is  not  constant.  This  condition  is  dependent  upon  the 
temperature,  the  pressure,  and  the  amount  of  water  vapor  present 
for  any  given  altitude. 

By  an  effect  of  refraction  the  day  in  middle  latitudes  is  lengthened 
several  minutes.  The  angular  diameter  of  both  the  sun  and  the 
moon  is  just  about  half  a  degree,  while  the  value  of  refraction  at  the 
horizon  is  a  little  more  than  half  a  degree.  The  result  is  that  both  the 
sun  and  the  moon  come  into  view  before  they  have  really  geometric- 
ally risen  above  the  horizon  and  are  still  visible  after  thay  have  really 

set. 

»  W.  I.  Milham,  "Meteorology." 

109 


110  MANUAL  OF   AEROGRAPH Y. 

Twinkling. — A  common  phenomenon  especially  noticeable  on  cold 
winter  nights  near  the  horizon  is  the  twinkling  of  the  stars.  It  con- 
sists of  an  apparent  change  in  position,  a  change  in  brightness,  and  a 
change  in  color. 

Since  the  atmosphere  consists  of  numerous  layers  and  pockets  of 
air  of  different  temperature,  moisture  content,  and  density,  which  are 
in  a  disturbed  condition  by  the  action  of  the  wind  upon  them,  the 
condition  of  the  air  through  which  a  ray  of  light  comes  to  the  observ- 
er's eye  changes  each  moment,  therefore  constantly  changing  the 
amount  of  refraction.  This  refraction  change  accounts  for  the  small 
change  of  position-. 

A  constant  change  in  brightness  is  caused  by  the  wind  wafting  the 
various  layers  and  pockets  of  air  past  the  line  of  sight  of  the  observer, 
concentrating  the  light  one  moment  while  the  next  moment  it  may  be 
diverted. 

The  rays  of  light  which  reach  the  observer  at  the  same  instant  may 
have  come  by  paths  of  slightly  different  length.  This  is  caused  by 
the  ether  waves  being  out  of  phase,  interfering,  and  causing  the  de- 
struction of  certain  wave  lengths  or  colors.  The  star  under  observa- 
tion will  appear  to  change  color  since  the  interference  will  be  different 
at  successive  moments. 

As  the  thickness  of  the  air  through  which  the  rays  of  light  come  is 
much  greater  near  the  horizon,  twinkling  is  more  marked  there. 
Except  when  near  the  horizon,  the  planets  seldom  appear  to  twinkle. 
This  is  because  they  are  disks  and  are  not  mere  points  of  light  like  the 
stars.  Each  point  on  the  disk  twinkles,  but  the  twinklings  do  not 
synchronize,  so  that  the  average  condition  of  the  whole  disk  is  much 
more  nearly  constant. 

Mirage. — The  conditions  necessary  for  this  phenomenon  are  a 
layer  of  very  warm  air  next  to  the  surface  of  the  ground  and  above 
it  a  layer  of  cold  dense  air.  To  an  observer  at  a  distance  from  this 
warm  layer,  the  rays  of  light  may  be  so  bent  by  refraction  that  a  total 
reflection  takes  place,  and  an  inverted  image  of  the  object  is  seen  as  if 
reflected  from  a  horizontal  body  of  water,  and  all  intervening  objects 
are  invisible.1 

A  mirage  usually  occurs  during  the  hot  hours  of  the  day  when  the 
air  is  quiet,  in  level  desert  regions  or  over  water  surfaces  near  the 
land.  Ths  position  of  the  observer  above  the  warm  layer  of  air  usu- 
ally makes  a  great  difference  in  the  appearance  of  the  mirage. 

Looming. — When  a  cold  dense  layer  of  air  is  next  to  the  surface 
of  the  ground  and  the  warmer  layer  of  air  is  above,  conditions  are 
favorable  for  another  phenomenon  called  looming.  The  rays  of  light 
passing  upward  from  an  object  may  be  so  bent  by  refraction  that  total 
reflection  again  takes  place,  and  an  inverted  image  above  the  object 

1  Milham,  "Meteorology." 


MANUAL   OF   AEROGRAPH  Y.  Ill 

i-  Been.  Nearer  objects  appear  raised  and  elongated,  while  objects 
below  the  horizon  may  be  brought  into  view.  Looming,  in  a  certain 
sense  the  opposite  of  mirage,  occurs  chiefly  over  the  ocean  near  the 
shore. 

Halos.  The  sun  and  moon  are  often  surrounded  by  rings  or 
circles  of  light  which  are  sometimes  colored  and  of  different  diameters. 
I'nless  of  great  intensity,  the  ring  appears  white  but  when  strongly 
developed  the  edge  nearest  the  sun  is  a  very  pure  red.  Orange, 
yellow,  and  under  favorable  conditions  green,  appear  outward  from 
the  red.  Green  is  always  faint  and  the  blue  which  follows  is  hardly 
recognizable.  Thus  the  ring  appears  white  on  its  outer  edge. 

Halos  are  due  to  the  refraction  and  reflection  of  the  rays  of  the  sun 
or  moon  by  ice  crystals.1  There  are  many  different  types  of  halo, 
14  having  been  enumerated.  The  most  common  form  is  the 
halo  of  22°.  A  close  relation  exists  between  this  halo  and  the  occur- 
rence of  cirro-stratus  cloud.  Halos  may  last  for  several  hours, 
depending  on  the  duration  of  the  cirro-stratus  cloud  sheet.  Since 
halos  depend  upon  the  prevalence  and  intensity  of  the  upper  cloud 
layer,  they  frequently  indicate  coming  storm  conditions. 

As  ;i  rule,  a  lunar  halo  is  so  little  colored  that  it  appears  essentially 
white. 

Another  ring  phenomenon  is  the  corona  which  is  frequently  seen 
about  the  moon  closely  surrounding  the  luminary.  Coronas  are 
caused  by  the  diffraction  of  light  by  water  drops.  The  smaller  the 
drops,  the  larger  the  ring.  When  several  rings  are  seen  at  the  same 
time,  water  drops  of  several  sizes  must  be  present. 

A  brownish  red  inner  ring  marks  the  corona  which  together  with 
the  bluish-white  inner  field  between  the  ring  and  the  source  of  light, 
forms  the  aureole.2  Should  other  colors  be  distinguishable,  they  fol- 
low the  brownish  red  in  the  order  from  violet  to  red.  Thus  the  order 
of  colors  in  a  halo  and  a  corona  are  opposite. 

As  coronas  are  diffraction  phenomena  they  show  the  sequence  of 
color  two  or  three  times  over.  This  can  never  be  the  case  with  a  halo. 

Several  other  optical  phenomena  are  occasionally  seen  that  are 
closely  associated  with  halos.  Sometimes  a  circle  of  white  light  may 
be  observed  parallel  to  the  horizon  and  at  the  altitude  of  the  sun. 
Where  this  circle  crosses  the  halos,  patches  of  light  appear  which  are 
known  as  mock  suns  or  sun  dogs.  Arcs  tangential  to  the  halos  and 
convex  to  the  sun  are  occasionally  seen.  These  phenomena  may  be 
explained  as  the  refraction  and  reflection  of  light  from  the  ice  particles 
which  are  either  oriented  in  a  definite  way  because  they  are  rising 
or  falling,  or  haphazard  in  arrangement. 

i  McAdie,  "The  Principles  of  Aerography." 
*  The  Observer's  Handbook,  1914. 


112  MANUAL  OF   AEROGRAPH Y. 

Rainbows. — A  rainbow,  or  arc  of  prismatic  colors,  is  formed  when 
the  sun  is  shining  and  at  the  same  time  it  is  raining  in  a  direction 
opposite  to  the  sun.  Violet,  blue,  green,  yellow,  orange,  and  red  are 
so  arranged  that  the  red  is  on  the  outside  and  the  violet  on  the  inside. 
The  radius  of  the  red  part  is  42°  2 '  while  that  of  the  violet  part  is  40° 
17'.  From  this  it  is  seen  that  no  rainbow  can  be  visible  if  the  sun  is 
more  than  42°  above  the  horizon.  The  sun,  the  observer's  eye,  and 
the  center  of  the  circle*  of  which  the  bow  is  a  part  must  always  be  in 
a  straight  line. 

The  maximum  number  of  rainbows  occur  in  the  later  part  of  a 
summer  afternoon  because  they  are  nearly  always  associated  with 
thunder  showers. 

The  rainbow  is  caused  by  the  refraction  and  reflection  of  sunlight 
in  the  falling  raindrops.  On  the  size  of  the  drops  and  their  uni- 
formity depends  the  pureness  of  the  rainbow's  color. 

Cloud  shadows. — When  the  sky  is  covered  with  broken  clouds, 
the  sun  is  allowed  to  shine  through  openings  in  the  clouds  and 
illuminate  the  moisture  and  dust  particles  beneath.  Light  streaks 
radiating  from  the  sun  and  extending  down  from  the  clouds  are  then 
seen.  Cloud  shadows  on  the  atmosphere  appear  as  dark  bands 
between  the  light  streaks  and  are  due  to  unillumination.  "The  sun 
drawing  water"  is  the  common  name  given  this  phenomenon.1 

The  coloration  of  the  sky. — A  cloudless  sky  appears  to  be  blue 
but  it  may  show  all  possible  gradations  ranging  from  a  deep  blue 
to  a  whitish  blue  shade.  The  clearer  the  sky  the  purer  and  more 
intense  the  blue. 

The  blue  color  of  the  sky  is  due  primarily  to  a  selective  scattering 
of  sunlight  by  the  numerous  particles  which  are  always  present  in 
the  atmosphere. 

Sunrise  and  sunset  colors. — When  the  sun  nears  the  horizon,  the 
thickness  of  the  atmosphere  through  which  the  light  comes  to  the 
observer  is  large,  so  that  the  wave  lengths  have  been  efficiently 
sorted,  and  only  the  red,  orange  and  yellow  light  gets  through.  The 
sun's  color  usually  becomes  yellow  or  orange  at  sunset  unless  the 
atmosphere  is  very  dusty  and  hazy,  when  it  may  be  decidedly  red. 

Changing  sunset  colors  and  glows  are  seen  from  the  time  the  sun 
approaches  the  horizon  until  it  is  some  16°  below. 

At  sunset,  due  to  the  colored  light  coming  from  the  west  and  re- 
flected to  the  observer  by  the  dust  particles  in  the  east,  colors  aie  also 
visible  in  the  eastern  sky.  As  the  sun  goes  farther  and  farther  below 
the  horizon  the  pink  twilight  arch  is  seen  rising  in  the  east.  A  blue- 
black  patch,  which  is  the  shadow  of  the  earth  on  its  atmosphere, 
appears  beneath  this  arch. 

iW.  I.  Milham,  "Meteorology." 


MANUAL  OF   AEROGRAPH Y.  113 

Sunrise  colors  are  practically  the  same  as  those  occurring  at  sunset, 
only  they  occur  in  the  inverse  order. 

Twilight. — -After  the  sun  has  gone  below  the  horizon  and  is  no 
longer  directly  visible  to  the  observer,  twilight  is  said  to  set  in.  It  is 
caused  by  the  reflection  and  diffraction  of  sunlight  by  the  numerous 
particles  in  the  Upper  atmosphere. 

Twilight  is  the  transition  period  between  daylight  and  darkness. 
The  duration  of  twilight  varies  at  different  phases  and  times  of  the 
year.  Twilight  is  stated  to  last  until  the  sun  has  gone  about  18°  below 
the  horizon. 

A.  S.  M. 
50821—18 8 


CHAPTER  XL 


INSTRUMENTS. 


115 


CHAPTER  XII. 


INSTRUMENTS. 

The  present  chapter  endeavors  to  give  a  clear  but  brief  description 
of  the  instruments  used  by  the  Naval  Aerographic  Stations,  their 
construction,  and  a  few  instructions  for  their  use  and  care. 

BAROGRAPH. 

The  barograph  gives  a  continuous  record  of  changes  in  atmospheric 
pressure.  The  motion  is  furnished  by  the  action  of  a  set  of  aneroid 
boxes,  indicated  on  a  chart  by  a  pen.  The  pen  rests  against  a  drum 
revolved  by  a  clock  within.  Over  the  drum  is  placed  a  recording 
sheet  upon  which  the  lines  are  traced. 

1.  Case. — The  case  is  approximately  25  by  15  by  12  centimeters 
high  and  is  substantially  made  of  wood  and  metal.     The  two  sides 
are  glazed  and  the  top  and  sides  are  removable  to  allow  access  to 
the   instrument   within.     Securely   fastened   to    the   bottom   is   an 
aneroid  barometer,  pen-controlling  mechanism,  and  a  cylinder  clock. 

2.  Barometer. — The  instrument  is  an  eight-cell  aneroid  barometer 
which  controls  the  pen  movement  through  a  series  of  levers. 

3.  Drum. — The  drum  is  approximately  30  centimeters  in  circum- 
ference and'  takes  a  record  sheet  9  centimeters  wide.     There  is  a 
flange  for  the  easy  setting  of  the  sheet  in  correct  alignment.     The 
sheet  is  held  in  place  by  rubber  bands. 

4.  Clock. — The  clock  is  of  the  cylinder  type,  set  inside  the  drum 
and  geared  to  it  in  such  a  manner  that  it  can  be  conveniently  re- 
moved for  adjustment.     It  turns  the  drum  surface  at  the  rate  of 
1  centimeter  per  hour  and  has  a  24-hour  movement. 

5.  Pen. — The  pen  is  metallic  and  glycerin  ink  is  used. 

Care  should  be  taken  to  adjust  the  barograph  from  time  to  time 
using  a  mercurial  barometer  as  a  standard. 

ANEMOSCOPE. 

The  anemoscope  gives  a  continuous  record  of  wind  direction.  The 
motion  in  the  anemoscope  is  furnished  by  a  wind  vane,  which  motion 
is  transferred  by  a  rod  through  the  roof  to  a  revolving  drum,  upon 
which  is  the  recording  sheet.  The  pen  rests  on  this  drum  and  moves 

downward  bv  clockwork. 

117 


118  MANUAL  OF   AEKOGRAPHY. 

The  anemoscope  used  by  the  Naval  Aerographic  Stations  has  the 
following  construction : 

1.  Case. — The  instrument  case  is  glazed  on  all  four  sides.     The 
front  is  hinged  to  provide  convenient  access  to  the  mechanism  within. 
Securely  fastened  to  the  bottom  is  a  support  for  a  record  drum  and 
the  clock  with  its  connecting  levers  and  pen  arm. 

2.  Wind  vane. — The  vane  is  approximately  1  meter  long  supported 
at  least  3  meters  above  the  roof.     It  is  properly  balanced  on  non- 
corrosive  bearings  and  is  weatherproof.     The  outside  sleeve  is  pro- 
vided with  guys  or  other  means  of  secure  fastening. 

3.  Drum. — The  drum  is  pivoted  to  the  wind  vane.     It  is  approxi- 
mately 35  centimeters  high  and  24  centimeters  in  circumference. 

4.  Clock. — The  clock  is  connected  to  the  pen  arm  through  a  pulley 
and  chain  in  such  a  way  as  to  move  pen  downward  at  the  rate  of 
1  centimeter  per  hour. 

5.  Pen. — The  pen  is  of  the  large  capacity  type  and  uses  glycerin 
ink. 

ANEMOBIAGRAPH. 

The  anemobiagraph  is  a  recording  instrument  combining  the 
speedometer  and  the  wind-direction  indicator.  That  used  at  the 
Naval  Aerographic  Stations  has  the  following  construction : 

1.  In  general. — The  instrument  consists  of  two  parts;  the  head 
and  the  recorder. 

2.  Head. — The  head  is  a  combination  wind  vane  and  phot  tube 
manometer.     From  it  two  simultaneous  records  are  made  on  the 
record  sheet;  one  indicating  wind  direction,  the  other  wind  force. 
The  head  is  of  such  a  size  as  to  assure  sufficient  power  to  control  the 
movement  of  the  anemoscope  pen  and  to  have  proper  air  ducts  to 
control  the  speedometer  float  and  its  pen.     The  head  is  carried  on  an 
annular  ball  bearing,  or  other  means  of  support  that  will  assure 
continual  freedom  of  movement. 

3.  Connections. — Change  of  direction  of  flow  is  transmitted  to  the 
recorder  by  a  rigid  steel  rod  and  cams,  and  variations  of  pressure 
(indicating   velocity   changes)    through   two  flexible  tubes  leading, 
respectively,  from  the  nozzle  and  staff  of  the  head.     The  outer  air 
support  is  approximately  1 1  meters  high,  is  air-tight,  and  is  anchored 
securely  to  a  concrete  base.     No  covering  is  required  indoor  over  the 
connection  to  the  recorder. 

4.  Recorder. — The  wind  direction  is  indicated  by  the  movement 
of  the  pen  actuated  by  the  vane.     The  wind  velocity  is  simultan- 
eously recorded  on  the  same  sheet  by  a  pen  whose  movement  is  varied 
according  to  the  relative  states  of  pressure  at  the  opening  in  the  head. 

5.  Drum. — The  pens  make  traces  on  the  record  sheet  on  the  drum, 
whose  circumference  is   about   60  centimeters   and  which   turns  2 


MANUAL   OF   AEBOGRAPHY.  119 

centimeters  ;m  hour.  The  record  sheet  is  1  f>  centimeters  wide. 
The  drum  has  a  flange  for  the  easy  «'tting  of  this  sheet  in  correct 
alignment. 

6.  ('/iK'k.     The  dock  is  of  the  cylinder  type  and  is  set  within  the 
drum  so  as  to  he  conveniently  removable. 

7.  Pens. — The  pens  are  metallic  and  use  glycerin  ink, 

THERMOGRAPHS. 

The  thermograph  gives  a  continuous  record  of  change  in  tempera- 
ture. The  motion  is  furnished  by  a  metallic  helix  which  curls  and 
uncurls  with  the  fall  and  rise  of  temperature  transferring  its  motion 
to  levers  which  operate  the  pen  arm.  The  pen  rests  on  a  drum 
which  revolves  under  it  by  clockwork. 

The  thermograph  used  at  the  Naval  Aerographic  Stations  has  the 
following  construction : 

1.  Case. — The  instrument  case  is  approximately  25  by   12  by  15 
cms.  high  and  is  made  substantially  of  wood  or  metal.     The  two 
sides  are  glazed  and  the  top  and  sides  are  removable  to  allow  v.< 

to  the  mechanism  within.  Securely  fastened  to  the  bottom  is  a 
metallic  thermometer  and  a  clock  base. 

2.  Thermometer. — The  thermometer  is  of  Bourdon  or  helical  metal 
type  with  the  free  end  connected  by  levers  to  operate  the  pen  arm. 

3.  Drum. — The  drum  is  approximately  30  cms.  in  circumference  to 
receive  a  24-hour  sheet  9  cms.  wide.     There  is  a  flange  for  the  easy 
setting  of  the  sheet  in  correct  alignment.     The  sheet  is  held  in  place 
by  rubber  bands. 

4.  Clock. — The  clock  is  of  the  cylinder  type,  set  inside  the  drum 
and  geared  to  it  in  such  a  fashion  that  it  is  conveniently  removable 
for  adjustment.     It  turns  the  drum  at  a  rate  of  one  cm.  per  hour  and 
has  a  24-hour  movement. 

5.  Pen. — The  pen  is  metallic  and  uses  glycerin  ink. 

ABSOLUTE    HYGROGRAPH. 

The  hygrograph  is  a  recording  hygrometer  which  gives  a  con- 
tinuous record  from  which  can  be  deduced  the  relative  humidity  of 
the  atmosphere  and  it  records  the  actual  weight  of  water  vapor  in 
the  air  per  unit  volume.  A  bundle  of  hair  cured  and  dried  so  as  to 
be  responsive  to  changes  in  moisture,  gives  the  percentage  of  satura- 
tion, while  the  dry  and  wet  thermometers  give  the  actual  and  sensible 
temperatures  from  which  may  be  deduced  the  absolute  humidity  for 
different  temperatures.  In  this  hygrograph  the  relative  humidity 
is  recorded  by  a  pen  which  takes  its  motion  from  connecting  levers 
attached  to  a  bundle  of  hair,  the  other  pens  are  controlled  by  the 


120  MANUAL   OF    AEROGRAPH Y. 

thermometers.     The  hygrographs  in  use  at  the  Naval  Aerographic 
Stations  are  of  the  following  construction. 

1.  Case. — The  instrument  case  is  30  by  18  by  15  cms.  high  and  is 
made  substantially  of  wood  or  metal.     The  sides  are  glazed  and  the 
ends  are  open,  protected  by  louvres  or  wire  screening.     The  whole 
upper  part  is  removable  to  allow  access  to  the  instrument  within. 
Supported  in  the  base  is  a  water  well,  and  fastened  on  the  base  are 
supporters  for  a  hygrometer,   a  wet  and  dry  thermometer,  and  a 
clock  and  drum  for  holding  a  record  sheet. 

2.  Well. — The  well  has  a  capacity  of  about  25  ccs.     It  is  concealed 
in  the  base  with  suitable  apertures  for  filling  and  for  receiving  the 
moistening  gauze  of  the  wet  thermometer. 

3.  Hygrometer. — The  hygrometer  is  of  the  hair  bundle  type  so 
connected  to  the  pen  that  the  latter  will  give  a  continuous  trace  of 
the  percentage  of  moisture  in  the  atmosphere.     Provision  is  made 
for  setting,  by  adjusting  the  distance  between  the  jaws  holding  the 
hair.     The  hairs  are  not  in  a  tight  bundle  but  separated  by  a  griddle. 

4.  Thermometers. — The  two  thermometers  are  identical  in  all  re- 
spects and  are  mounted  on  the  same  axis.   They  are  metallic,  helical 
in  form,  with  an  outside  diameter  of  about  two  cms.  and  have  four 
turns  between  the  point  of  support  and  the  attachment  of  the  pen 
controlling  levers. 

5.  Pen  operating  mechanism. — The  pens  are  controlled  in  such  a 
manner  that  their  traces  are  directly  above  one  another.     The  pen 
movement  is  vertical,  not  curved. 

6.  Drum. — The  drum  is  approximately  30  cms.  in  circumference  to 
receive  a  24-hour  record  sheet,  12  cms.  in  width.     There  is  a  flange 
for  the  easy  setting  of  the  sheet  in  correct  alignment.     The  sheet  is 
held  in  place  by  rubber  bands. 

7.  Clock. — The  clock  is  of  the  cylinder  type,  set  inside  the  drum  and 
geared  to  it  in  such  a  manner  as  to  be  easily  removable  for  adjustment. 

8.  Pens. — The  pens  are  metallic  and  use  glycerin  ink. 

SUNSHINE    RECORDER. 

The  sunshine  recorder  (Campbell-Stokes)  is  an  instrument  which 
gives  a  record  of  the  amount  of  sunshine.  The  sunshine  recorder 
used  by  the  Naval  Aerographic  Stations  has  the  following  construction : 

1.  General  description. — The  instrument  is  substantially  made  to 
withstand  continued  exposure  to  the  weather.     It  consists  of  a  metal 
frame  supporting  a  lens  and  bowl  for  the  record  sheets. 

2.  Lens. — The  lens  is  crown  glass  with  a  standard  diameter  of 
10  cms.,  a  focal  length  of  7.5  cms.,  and  weighing  1,360  grams. 

3.  Bowl. — The  bowl  is  made  to  accommodate  three  cards,  one  for 
the  summer  sun,  one  for  the  equinoctial  sun,  and  one  for  the  winter 


MANUAL    OF    AEROGRAPH Y. 


121 


sun.  There  is  provision  for  adjustment  to  accommodate  a  latitude 
change  of  20°.  The  diameter  of  the  bowl,  measured  between  the 
6  o'clock  marks,  is  approximately  14.5  cms.  and  the  radius  25  cms. 
The  belt  from  which  the  bowl  is*  made  is  so  cut  that  when  the  bowl 
is  adjusted  for  its  mean  latitude  the  plane  of  the  top  cut  is  nearly 
horizontal. 

BALLOON  EQUIPMENT. 

Balloons  are  used  for  determining  the  direction  of  air  flow;  also 
temperature,  humidity,  and  pressure  at  various  levels.  Sounding 
may  be  defined  as  the  making  of  successive  sights  at  regular  intervals 
by  means  of  the  theodolite.  With  pilot  balloons  having  an  ascension 
speed  nearly  constant  (previously  determined),  one  can  determine 
the  successive  positions  of  the  balloon  in  space  and  can  deduce  from 
that  the  direction  of  the  wind  and  its  approximate  speed  at  different 
altitudes. 

GENERAL    EQUIPMENT. 

1.  Balloons. — There  are  two  sizes,  one  to  inflate  to  60  cms.  and 
the  larger  to   about  84  cms.     They  are  made  of  rubber  and    are 
equipped  with  automatic  valves.     They  are  known,  respectively,  as 
small  testers  and  pilot  balloons. 

2.  Hydrogen. — The  hydrogen  supply  comes  in  commercial  iron 
tanks,  and  there  is  a  suitable  equipment  of  valves,  pressure  gauges, 
and  tubing. 

3.  Theodolites. — The  theodolites  are  similar  to  those  developed  for 
the  use  of  the  Canadian  Meteorological  Service.     Two  are  required 
for  simultaneous  use. 

It  has  been  found  that  these  balloons  can  not  be  safely  inflated  to 
lift  more  than  an  extra  10  grams. 

It  is  believed  that  most  of  the  balloon  fillers  supplied  weigh  15 
grams,  but  in  case  of  possible  variations  the  following  table  is  given: 


Weight  of 
filler  and 
weight  -;  to 
be  left  on. 

Weights  to 
be  left  on. 

Grams. 
18.0 
25.5 
33.5 
42.0 
51.0 
60.5 

Grams. 
15.0 
22.0 
29.5 
37.5 
46.0 
55.0 

It  is  suggested  that  the  weight  to  be  left  attached  when  the  balloon 
is  released  should  consist  of  a  piece  of  lead  wire. 


122  MANUAL   OF    AEROGRAPH Y. 

LIGHTNING   RECORDER. 

A  ceraunograph,  or  lightning  recorder,  is  a  device  to  provide  a 
warning  of  the  approach  of  a  storm  at  a  time  sufficiently  far  in 
advance  to  increase  the  accuracy  and  value  of  local  weather  forecasts. 

Practically  all  summer  storms  are  accompanied  by  electrical  dis- 
turbances in  the  ether.  By  use  of  antennae,  some  of  these  radia- 
tions may  be  intercepted  and  by  a  suitable  apparatus  be  made  to 
give  an  indication  of  not  only  the  presence  but  also  the  relative 
proximity  of  the  storm. 

The  necessary  apparatus  to  construct  a  ceraunograph  are  an 
aerial,  coherer,  decoherer,  relay,  recorder  and  batteries. 

The  action  of  the  instrument  is  based  upon  the  effect  that  high- 
tension  electric  waves,  in  free  air,  have  upon  a  coherer.  When 
lightning  occurs  in  the  vicinity  of  the  coherer,  some  of  the  electric 
waves  travelling  through  the  ether  affect  the  filings,  causing  them 
to  cohere.  This  decreases  their  electrical  resistance,  thereby  allow- 
ing a  local  battery  current  to  operate  a  relay  in  circuit  with  the 
filings,  which  in  turn  operates  a  decoherer.  The  decoherer  separates 
the  filings  and  restores  them  to  their  original  condition  and  at  the 
same  time  records  the  passage  of  the  electrical  waves. 

Ceraunographs  are  capable  of  giving  a  warning  of  coming  lightning 
from  a  few  hours  to  20  hours  in  advance,  the  strength  and  frequency 
of  the  signals  not  only  denoting  whether  the  storm  is  approaching 
or  receding,  but  also  if  it  is  increasing  or  decreasing  in  energy. 


MAXIMUM    AND    MINIMUM    THERMOMETERS. 


The  maximum  thermometer  is  designed  to  record  the  highest  tem- 
perature experienced  during  a  given  period.  Two  forms  of  instru- 
ment are  in  common  use.  Both  are  mercurial  thermometers.  In 
the  Negretti  &  Zambra  patterns,  adopted  by  the  Meteorological 
Office,  the  tube  is  greatly  constricted  just  above  the  bulb.  It  is 
hung  nearly  horizontally  with  the  bulb  end  slightly  lower  than 
the  other.  As  the  temperature  rises  the  mercury  expands  and  is 
forced  past  the  constriction,  but,  when  a  subsequent  fall  of  tempera- 
ture causes  a  contraction  of  the  mercury,  the  thread  breaks  at  the 
constriction,  so  that  its  upper  end  remains  in  position  to  register  the 
highest  temperature  reached. 

Phillips's  pattern  is  also  hung  horizontally.  In  it  there  is  no  con- 
striction in  the  tube,  but  a  small  air  bubble  is  placed  in  the  mercury 
thread  near  the  bulb.  As  the  temperature  falls  the  part  of  the  mer- 
cury beyond  the  bubble  is  not  drawn  back  toward  the  bulb  and 
thus  the  end  of  the  mercury  column  marks  the  highest  temperature 
reached. 

i  The  Observer's  Handbook. 


MANUAL   of    \K.i;<M,-;.U'llY.  123 

The  minimum  thermometer  records  the  lowest  reading  experi- 
enced in  a  general  interval.  The  most  common  type  of  instrument 
is  a  spirit  thermometer,  having  a  small  index  in  the  stem.  It  is  hung 
like  the  maximum  thermometer.  As  the  temperature  falls,  the  index 
is  carried  toward  the  bulb  by  the  spirit,  but  if  the  latter  subsequently 
expands  in  consequence  of  a  rise  of  temperature,  it  flows  past  the 
index,  which  is  left  in  position  to  indicate  the  lowest  temperature 
reached. 

Defects  of  maximum  thermometers. — Maximum  thermometers  are 
subject  to  two  defects: 

(1)  The  mercury  may  recede  from  its  maximum  position  when  the 
temperature  falls  belowr  the  maximum  to  a  greater  or  less  extent.     The 
observer   should    accordingly    test   his   instrument    occasionally    by 
gently  heating  it  and  noting  whether  the  mercury  column  retains  its 
position  in  the  tube. 

(2)  The  mercury  may  slip  forward  when  the  instrument  is  brought 
into  a  horizontal  position  after  setting. 

Both  these  defects  may,  in  most  cases,  be  remedied  by  altering  the 
inclination  at  which  the  instrument  hangs. 


SOLAR    RADIATION    THERMOMETER. 


(Black  bulb  in  vacuo.) 

For  obtaining  some  indication  of  the  intensity  of  the  sun's  radia- 
tion, a  maximum  thermometer  having  the  bulb  and  1  inch  of  the 
stem  coated  with  dull  lampblack  is  used.  The  whole  is  inclosed  in 
a  glass  jacket  which  is  exhausted  of  air. 

The  site  for  exposure  may  be  near  the  thermometer  screen.  The 
proximity  of  trees,  buildings,  etc.,  must  be  avoided.  The  instru- 
ment is  fixed  on  a  wooden  stand  at  the  same  height  above  the  ground 
as  the  thermometers  in  the  screen  (4  feet).  The  bulb  must  be  freely 
exposed  to  the  sun,  and  hence  the  tube  should  be  directed  from  east 
to  west. 

The  difference  between  the  maximum  shown  by  the  " black  bulb" 
and  the  maximum  reading  in  the  thermometer  screen  is  usually 
regarded  as  an  index  of  the  intensity  of  solar  radiation. 

Readings  are  taken  once  a  day  only,  at  9  p.  m.  The  method  of 
off  setting  is  precisely  similar  to  that  used  in  the  case  of  the  maximum 
thermometer. 

TERRESTRIAL  RADIATION  THERMOMETER. 

(Grass  minimum.) 

For  estimating  the  effect  of  radiation  from  the  earth's  surface  at 
nighttime,  a  minimum  thermometer  exposed  freely  on  a  grass  surface 
is  used.  To  secure  greater  sensitiveness,  the  wooden  mounting  of 

1  The  Observer's  Handbook. 


124  MANUAL   OF    AEROGRAPH Y. 

the  ordinary  minimum  thermometer  is  dispensed  with.  With  the 
same  object,  various  forms  of  bulb  have  been  suggested.  An  outer 
glass  case  is  generally  sealed  round  the  stem  of  the  instrument  to 
protect  the  tube  and  prevent  condensation  of  the  spirit  in  the  upper 
end. 

The  thermometer  should  be  supported  on  two  Y-shaped  pieces  of 
wood  at  a  height  of  1  or  2  inches  above  the  ground,  which  should  be 
covered  with  short  grass.  Care  should  be  taken  that  the  bulb  does 
not  touch  the  supports,  as  this  would  diminish  the  sensitiveness.) 
The  proximity  of  walls,  trees,  benches,  etc.,  should  be  avoided. 

When  the  ground  is  covered  with  snow,  the  thermometer  should 
be  supported  immediately  above  the  surface  of  the  snow,  as  near  to 
it  as  possible  without  actually  touching  it. 

Hour  of  reading  and  setting. — The  hour  for  reading  and  setting 
the  grass  minimum  thermometer  raises  an  important  question.  The 
climatic  fact  which  the  observations  should  supply  is  the  number  of 
nights  of  ground  frost.  If  9  a.  m.  be  selected  as  the  hour  for  setting, 
it  will  frequently  happen  that  the  reading  to  which  the  instrument 
is  set  will  be  the  minimum  for  the  ensuing  24  hours,  and  if  the  value 
happens  to  be  below  30°  F.  (or  30.4°  F.  if  the  thermometer  is  read  to 
tenths  of  a  degree)  the  statistics  may  show  more  "days  of  ground 
frost"  than  there  were  nights  of  frost.  At  stations  where  evening 
readings  are  taken  the  thermometer  should  be  read  and  set  at  the 
hour  of  evening  observation.  At  stations  at  which  observations  are 
taken  once  a  day  only  arrangements  should  be  made  for  setting  the 
instrument  in  the  afternoon  or  evening;  the  reading  may  be  taken 
in  the  morning. 

Bubbles  in  stem. — The  protection  of  the  stem  of  a  grass  minimum 
thermometer  by  an  outer  jacket  is  not  always  sufficient  to  prevent 
the  spirit  separating  into  detached  portions.  During  great  cold  and 
also  when  exposed  to  strong  sunshine  grass  minimum  thermometers 
are  very  liable  to  the  development  of  bubbles  on  the  bulb  or  stem  or 
to  the  condensation  of  drops  of  spirit  in  the  upper  part  of  the  stem- 
Great  care  must  be  taken  to  avoid  errors  due  to  either  of  these  causes. 
In  summer  it  is  advisable  to  place  the  instrument  indoors  during  the 
daytime  when  the  sun  is  very  powerful.  It  should  be  kept  in  a 
vertical  position,  bulb  downward,  while  not  in  use. 

Spirit  thermometers  should  be  regularly  examined  for  the  presence 
of  bubbles  in  the  stem  or  bulb,  or  of  drops  of  liquid  in  the  upper  part 
of  the  stem,  or  in  the  small  bulb  at  its  end.  To  remedy  this  defect 
when  present,  hold  the  thermometer  with  the  bulb  downward  and  the 
tube  vertical  and  jolt  the  bulb  end  of  the  frame,  or  if  there  is  no  frame, 
the  hand  holding  the  thermometer,  gently  against  a  soft  pad,  keeping 
the  instrument  vertical  all  the  time.  One's  knee  or  a  thickly  folded 
tablecloth  forms  a  suitable  pad  to  prevent  the  jar  being  too  severe. 


MAM  AL   OF   AEROGRAPH Y.  125 

By  repeating  this  treatment  several  times  detached  globules  of  spirit 
may  be  made  gradually  to  approach  the  main  bulk  of  spirit,  and 
ultimately  the  whole  thread  becomes  continuous.  After  all  visible 
drops  or  bubbles  have  been  removed  in  this  way  the  thermometer 
should  be  left  for  a  short  time  in  a  vertical  position,  bulb  downward, 
to  allow  any  liquid  which  may  have  collected  on  the  walls  of  the  tube 
to  drain  down  to  the  main  column.  In  hot  weather  especially  it  is 
advisable  to  take  suitable  opportunity  for  keeping  spirit  thermom- 
eters in  this  position  in  cold  water  for  some  hours  in  the  daytime. 

Occasionally  the  thread  of  a  mercury  thermometer  gets  broken; 
the  defect  may  generally  be  remedied  by  jolting  as  described  above. 


SCREEN.1 


The  dry  bulb,  wet  bulb,  and  the  maximum  and  minimum  ther- 
mometer are  exposed  in  a  screen  of  the  following  patterns.  The 
screen  for  this  climate  in  general  use  is  the  Stevenson  screen.  It  is  a 
box  or  cupboard  with  double  louvred  sides.  It  has  the  following 
construction : 

Material. — The  screen  is  to  be  constructed  throughout  of  the  best 
yellow  pine  and  all  its  parts  put  together  with  tenons,  mortises,  and 
brass  screws,  with  the  exception  of  the  louvres,  which  are  fastened 
together  and  secured  in  place  by  brass  rivets. 

Framework. — This  consists  of  four  corner  posts,  connected  above 
and  below  by  rails. 

Louvres. — The  screen  has  double  louvres.  The  outer  louvres  are 
slipped  into  shallow  grooves  cut  in  the  inner  sides  of  the  four  corner 
posts  of  the  screen  at  an  angle  of  45°  and  one-half  inch  apart  measured 
square  to  the  groove.  At  the  two  back  inner  corners  of  the  screen 
the  louvres  are  mitred  roughly.  The  outer  edges  of  the  outer  louvres 
are  made  flush  with  the  corner  posts;  the  inner  louvres  project 
beyond  the  posts  into  the  screen. 

Door. — The  door  forms  one  of  the  longer  sides  of  the  screen.  It  is 
a  rectangular  frame  fitted  with  double  louvres  similar  to  those 
described  above.  It  is  hung  by  its  outer  bottom  edge  to  the  lower 
front  rail  by  two  strong  hinges  and  closes  with  its  outer  surface  flush 
with  the  corner  posts. 

Bottom  of  screen. — This  is  formed  by  three  boards,  arranged  as 
follows :  The  center  or  upper  board  is  set  into  the  end  rail  of  the  frame, 
so  as  to  be  flush  with  the  top  of  the  lower  side  rails,  while  the  other 
two  are  screwed  to  the  under  sides  of  the  end  rails  in  such  a  way 
that  one  overlaps  the  back  edge  and  the  other  by  the  same  amount 
the  front  edge  of  the  center  board  above. 

Roof. — The  roof  is  to  be  double.  The  inner  roof  is  to  be  formed  by 
a  board  one-half  inch  thick,  resting  upon  the  upper  rails  and  cut 

i  Adopted  from  The  Observer's  Handbook. 


126  MANUAL  OF   AEROGRAPH Y. 

away  to  receive  the  corner  posts.  It  has  holes,  each  an  inch  in  diam- 
eter, drilled  in  it  at  equal  distances  all  around,  the  centers  of  the  holes 
being  3^  inches  from  the  outer  edge. 

The  outer  roof  is  to  be  a  1-inch  board  screwed  on  to  the  top  of  the 
corner  posts,  and  also  to  a  narrow  bearing  of  wood,  running  across 
the  center  of  the  inner  roof  from  front  to  back.  The  underside  of 
the  outer  roof  is  H  inches  above  the  inner  roof  in  front,  but  only 
one-half  inch  above  it  at  the  back,  and  it  must  project  beyond  the 
sides  of  the  screen  all  around.  A  clear  space  will  thus  be  left  between 
the  two  roofs,  which  in  the  front  will  measure  1 J  inches,  and  in  order 
to  partly  close  this  a  small  lathe,  three-fourths  inch  wide  and  one- 
half  inch  thick,  is  to  be  fastened  across  the  center  of  it. 

Position  of  thermometer. — The  upright  for  the  dry  and  wet  bulb 
thermometers  is  screwed  to  the  back  of  the  middle  bottom  board. 
The  two  uprights  across  which  the  maximum  and  minimum  ther- 
mometers are  hung  are  screwed  to  the  front  of  the  same  bottom 
board.  The  upper  ends  of  these  uprights  are  screwed  to  fillets 
attached  to  the  underside  of  the  inner  roof.  If  the  dry  and  wet 
bulb  thermometers  are  already  fixed  upon  a  frame,  the  frame  may  be 
hung  upon  the  upright;  but  if  the  thermometers  are  separate,  two 
strips  of  wood  are  fixed  to  the  upright,  at  right  angles  to  it.  In 
these  cross  pieces  grooves  are  cut,  which  in  the  case  of  the  upper 
one  are  right  across  the  strip,  but  in  the  lower  stops  short  of  the 
bottom.  These  grooves  are  wider  at  the  back  than  at  the  front. 
The  thermometer  scale  rests  on  the  bottom  of  the  lower  groove, 
and  the  instrument  is  then  secured  in  its  place  by  means  of  small 
brass  buttons,  which  are  turned  over  the  outside  edge  of  the  ther- 
mometer scale,  and  thus  hold  it  firmly  in  the  groove. 

Painting. — Previous  to  their  being  put  together  all  the  different 
parts  have  two  coats  of  white  lead  paint;  and  when  completed  the 
whole  screen  receives  a  finishing  coat  composed  of  white-zinc  paint 

and  copal  varnish. 

NEPHOSCOPE. 

The  nephoscope  is  used  in  determining  the  direction,  height  and 
velocity  of  clouds.  It  consists  of  a  graduated  black  glass  mirror,  so 
mounted  as  to  allow  of  accurate  leveling,  and  an  adjustable  pointer. 

The  instrument  in  use  by  the  Naval  Aerographic  Stations  has  the 
following  construction : 

1.  Mirror. — The  mirror  has   three  equidistant  concentric  circles 
engraved  on  its  surface,  as  well  as  the  usual  radial  lines,  and  a  trans- 
parent space  through  which  the  position  of  a  magnetic  needle  mounted 
within  the  mirror  case  may  be  seen. 

2.  Mounting. — Mounted  in  the  same  plane  as  the  mirror  and  in  the 
same  plane  with  it  is  an  adjustable  brass  pointer.     A  magnetized 


MANUAL   OF   AEROGRAPH Y.  127 

compass  needle  is  pivoted  under  the  center  of  the  mirror  with  its  pin 
supported  by  the  mirror  case. 

3.  Mirror  case. — The  mirror  case   is  supported  and  balanced   so 
that  it  will  rest  horizontally  even  though  the  instrument  base  is  rest- 
ing on  an  irregular  surface.     The  bearings  are  adjustable,  and  the 
instrument  made  of  brass  or  of  aluminum. 

4.  Case. — The  carrying  case  is  made  as  small  as  possible  and  prac- 
tically all  of  wood.     The  cover  is  hinged  and  a  socket  provided  in 
which  the  instrument  may  be  screwed  in  order  to  make  use  of  the 
case  as  a  base.     Inside  are  plush-lined  pockets  to  fit  the  various  parts 
of  the  instrument. 

The  method  of  observing  is  as  follows: 

The  observer  stations  himself  in  such  a  position  that  the  image  of  the  cloud  in  the 
glass  and  the  central  point  of  the  mirror  are  seen  in  the  same  straight  line.  He  then 
rotates  the  pointer  and  adjusts  its  length  until  its  tip  is  also  brought  into  this  straight 
line.  This  done,  he  moves  his  head  so  as  to  keep  the  cloud  image  and  the  tip  of  the 
pointer  in  coincidence  and  notes  the  radius  along  which  the  image  appears  to  travel. 
This  radius  marks  the  direction  of  cloud  drift.  A  compass  needle  mounted  below  the 
disk  enables  the  observer  to  identify  this  direction;  the  variation  of  the  compass,  how- 
ever, must  always  be  noted  and  duly  allowed  for  in  all  observations  of  direction. 

The  velocity-height  ratio  of  the  cloud  may  be  determined  by  noting  the  number  of 
seconds  required  for  the  image  to  travel  from  the  center  of  the  mirror  to  the  first  circle 
or  one  circle  to  the  next.  If  "a  "  be  the  radius  of  the  inside  circle,  "6  "  be  the  height 
of  the  tip  of  the  pointer  above  the  reflecting  surface  and  "t"  be  the  time  required  for 
the  cloud  image  to  traverse  the  distance  "a"  (both  "a"  and  "b"  being  measured  in 
millimeters),  the  value  of  the  velocity-height  ratio,  as  it  would  appear  to  an  observer 
at  a  point  on  the  surface  vertically  below  the  cloud  is  given  by  the  equation — velocity- 
height  ratio=a  divided  by  bt.] 

RAIN    GAUGE. 

A  rain  gauge  is  an  instrument  for  ascertaining  the  amount  of 
rainfall.  The  recording  rain  gauge  in  use  at  the  Naval  Aerographic 
Stations  has  the  following  construction: 

1.  Construction. — The  outer  casing  is  copper  of  sufficient  weight 
to  avoid  denting  and  is  weather-proofed.     Over  the  top  is  a  remov- 
able cover.     Under  the  inner  rim  of  the  latter  is  a  balance  and 
train  of  levers  to  operate  a  pen.     Also  supported  on  the  bottom  is  a 
clock  and  drum  receiving  a  record  sheet.     On  the  front  of  the  lower 
part  of  the  outer  casing  is  an  access  door,  which  is  glazed. 

2.  Cover. — The  cover  of  the  same  weight  as  the  casing  (copper) 
fits  tightly,  tapering  inward  to  a  standard  brass  rain-gauge  ring  at 
the  lip  of  the  opening.     Its  inner  collar  is  long  enough  to  project 
inside  the  rim  of  the  receiver,  and  its  outer  collar  5  or  6  centimeters 
over  the  casing  of  the  apparatus. 

i  The  Observer's  Handbook. 


128  MANUAL  OF   AEROGRAPH Y. 

3.  The  receiver. — The  receiving  can  is  cylindrical  and  of  sufficient 
capacity   to   record    a   15    centimeter  rainfall   in   24  hours  without 
overflow. 

4.  Balance,   etc. — The  receiver  will  rest  on   a  platform  counter- 
balanced by  a  weight  or  spring  and  which  will    move    downward 
according  to  the  weight  of  the  precipitation  in  the  can.     This  move- 
ment raises  a  pen  arm  by  means  of  a  suitable  train  of  levers,  indi- 
cating on  a  drum  the  amount  of  rainfall. 

5.  Drum. — The  drum  is  approximately  30  centimeters  in  circum 
ference  to  receive  a  24-hour  record  sheet  9  centimeters  wide.     Thei  c 
is  a  flange  for  the  easy  setting  of  this  sheet   in    correct    alignment 
The  sheet  is  held  in  place  by  rubber  bands. 

6.  Clock. — The  clock  is  of  the  cylinder  type,  set  inside  the  drum  and 
geared  to  it  in  such  a  manner  that  it  will  be  easily  removable  for 
adjustment.     It  turns  the  drum  surface  at  a  rate  of  1  centimeter 
per  hour. 

7.  Pen. — The  pen  is  to  be  metallic,  using  glycerin  ink. 

AEROGRAPH. 

The  aerograph  is  a  combination  of  the  thermograph,  barograph, 
and  hygrograph,  all  making  simultaneous,  continuous  records. 
The  Naval  Aerographic  Stations  are  using  an  aerograph  of  the  follow- 
ing construction : 

1.  In  general. — For  airplane  work  the  instruments  are  as  light 
and  compact  as  possible. 

2.  Case. — The  case  is  approximately  6  by  30  by  60  centimeters, 
made   of  sheet   aluminum   properly    reinforced   at    the    edges    and 
corners.     There  are  lugs  on  each  side  for  making  the  instrument  fast. 
There  are  one  or  more  transparent  panels  of  unbreakable  material  on 
the  sides.     The  instrument  is  entirely  inclosed  and  the  doors  securely 
fastened.     Proper  ventilation  to  the  interior  is  provided. 

3.  Instruments. — The  clock  is  as  light  as  possible  for  accuracy  of 
rate.     It  will  drive  the  paper  drum  or  drums  and  also  make  time 
marks  on  the  record.     The  thermograph  is  a  bi-metal  helix,  with  its 
free  end  controlling  a  pen.     The  barograph  is  an  aneroid  of  at  least 
six  cells.     The  hydrograph  consists  essentially  of  a  bundle  of  pre- 
pared hairs,   exposed  in  the  line  of  most  direct  ventilation.     To 
increase  the  sensitiveness  the  hairs  are  supported  by  a  griddle.     The 
pens  are  supported  on  cords,  rather  than  on  pivoted  lever  arms,  in 
order  that  their  motion  may  be  directly  vertical.     The  paper  carrier 
is  easily  removable.     All  possible  parts  are  cut  from  aluminum,  and 
the  others  made  of  well-machined  brass,  as  light  as  safety  permits. 

W.  F.  P. 


CHAPTER  XIII. 


SIGNALS. 


50821—18 9 


129 


CHAPTER  XIII. 


SIGNALS. 

Telegraphic  reports  of  meteorological  observations  are  to  be  for- 
warded according  to  the  following  code: 

UNITED    STATES   NAVY   WEATHER   REPORT   CODE. 

Letters  indicate  direction  Jrom  which  wind  and  clouds  are  moving 
as  follows: 

[Cloud  definitions  according  to  the  International  System.] 


Code  letter. 

Direction. 

Cloud  type. 

Code  letter. 

Direction. 

Cloud  type. 

B. 

0°  north.. 

Cloudless  . 

M. 

180°  south... 

Strato-cumulus 

c 

30° 

Fog 

N 

210° 

Alto-stratus 

D... 

60° 

Stratus. 

P... 

240° 

Alto-cumulus. 

P 

90°  east 

Cvi  mi  i  lo-ni  nihus  , 

R 

270°  west 

Cirro-ctimulus 

G... 

120° 

Cumulus. 

S. 

300° 

Cirro-stratus. 

H 

150° 

Nimbus 

T 

330° 

Cirrus. 

Wind  tendency: 

(V)  Veering  or  (B)  backing. 

(I)  .Increasing  or  (D)  decreasing. 

(C)  Constant  or  (G)  gusty. 
Barogram  description: 

(R)  Rising  or  (F)  falling. 

(S)   Smooth  or  (J)  jerky. 

(     )  Number  of  hours  since  recent  change  in  direction. 

The  message  is  to  be  sent  in  eight  parts  with  an  interval 
(•  —  •—•)  between  each  part  in  the  following  order: 

1 .  Place  and  day  of  the  month:  Chatham  8. 

2.  Time,  a.  m.  or  p.  m.:  Ten  A. 

3.  Wind  direction  and  its  velocity  in  meters  per  second:  Fo.     When  calm  send 
word  "calm." 

4.  Pressure  in  kilobars  above  (A)  or  below  (B)  1,000  kbs.:  A7. 

5.  Temperatures  in  degrees  absolute:  A284. 

6.  Cloud  type,  direction  of  movement,  and  velocity:  SR-5. 
1 .  Wind  tendency  and  barogram  description:  DCS.;. 

8.  Any  additional  information. 

STORM    SIGNALS. 

As  far  as  possible  information  concerning  weather  conditions 
should  be  obtained  from  the  nearest  weather  station  by  telephone  or 
telegraph. 

Notice  of  threatening  atmospherical  disturbances  in  the  vicinity  of 
the  coastal  station  should  be  displayed  as  a  warning  to  pilots. 

As  there  are  several  signal  systems  in  use  throughout  the  world, 
those  in  present  use  in  the  United  States,  Great  Britain,  and  France 
are  quoted  below. 

131 


132 


MANUAL   OF   AEEOGEAPHY. 


UNITED    STATES. 

Day  apparatus. — Two  red  flags  with  black  center,  and  red  or  white  pennant  dis- 
played at  masthead,  one  above  the  other. 
Information  given : 

Pennant:  Moderately  strong  winds. 
Flag:  Markedly  violent  storm. 

Combination  of  flag  and  pennant  indicates  quadrant  from  which  the  wind  may 
be  expected,  and  also  whether  the  center  is  approaching  or  receding. 


Storm  center  has  passed. 

Storm  center  approaching. 

NW. 

SW. 

NE. 

SE. 

White 
pennant  over 
flag. 

Flag  over 
white 
pennant. 

Red  pennant 
over  flag. 

Flag  over 
red  pennant. 

Two  red  flags. — Expectation  of  a  very  dangerous  storm  or  hurricane. 

Night  apparatus. — Red  light  and  white  light. 

Information  given. — Red  light  indicates  easterly  winds;  white  light  below  red, 
westerly  winds;  no  night  hurricane  warnings. 

Duration  of  signal. — Signals  remain  displayed  for  24  hours  from  the  time  specified 
in  the  order  to  hoist,  change,  or  continue  them,  and  no  longer,  unless  a  subsequent 
telegram  is  sent  ordering  them  down. 

Sometimes  a  cautionary  signal  is  displayed  indicating  that  condi- 
tions dangerous  to  small  craft  but  not  unfavorable  to  large  craft  may 
be  expected.  The  warning  consists  of  a  signal  pennant,  either  white 
or  red,  depending  whether  the  winds  are  to  be  easterly  or  westerly, 
respectively. 

GREAT    BRITAIN    AND   IRELAND. 

Day  apparatus. — Cone  hoisted  at  yardarm  or  masthead. 

Information  given. — Expectation  of  strong  wind  or  gales  from  north  to  east,  backing 
through  north  (point  upward),  from  south  or  east  veering  through  south  to  northwest 
(point  downward). 

Night  apparatus. — Three  lanterns  of  any  color,  preferably  red,  and  cross  bar. 

Information  given. — Same  as  by  day.     A  triangle  of  lanterns  replace  the  cone. 

NOTE. — Night  signals  are  exhibited  at  very  few  stations  in  the  British  Isles.  At 
some  stations  the  cone  is  hoisted  where  it  is  illuminated  by  artificial  light.  A  cone 
is  3  feet  high  and  3  feet  wide  at  the  base,  is  made  of  black  canvas,  and  has  the  appear- 
ance of  a  triangle  when  hoisted . 

Duration  of  signal. — From  time  of  receipt  of  telegram  till  8  p.  m.  of  the  following 
day.  Orders  to  prolong  or  lower  the  signal  are  dispatched  if  necessary. 

FRANCE. 

Apparatus. — Two  cones. 

Information  given. — Single  cone,  point  upward:  Gale  commencing  with  wind  in  the 
northwest  quadrant.  Single  cone,  point  downward:  Gale  commencing  with  wind  in 
the  southwest  quadrant.  Two  cones,  one  above  the  other,  both  points  upward:  Gale 
commencing  with  wind  in  northeast  quadrant.  Two  cones,  one  above  the  other, 
both  points  downward:  Gale  commencing  with  wind  in  southeast  quadrant.  Two 
cones  with  their  bases  together:  Hurricane. 

XOTK. — The  distance  between  two  cones  hoisted  in  vertical  line  should  be  the 
same  as  the  length  of  the  slant  side  of  the  cones. 

Night  signals:  None.  A.  S.  M. 


CHAPTER  XIV. 


FORECASTING. 


133 


CHAPTER  XIV. 


FORECASTING,  PART  1,  STORMS.  ' 

Types  of  storm's. — It  is  the  practice  of  the  Forecast  Division  of 
the  Weather  Bureau  to  classify  storms  after  the  region  where  first 
charted.  Thus,  one  of  the  most  frequent  types  is  known  as  the 
"Alberta,"  because  it  is  definitely  charted  hi  that  territory.  The 
system  has  its  defects,  and  with  each  enlargement  of  the  area  of  ob- 
servation some  modification  of  the  place  of  origin  becomes  apparent. 
In  a  recent  publication  by  Bowie  and  Weigh  tm  an  2  the  types  are 
given  as  Alberta,  North  Pacific,  South  Pacific,  Northern  Rocky 
Mountain,  Colorado,  Texas,  East  Gulf,  South  Atlantic,  Central,  and 
West  Indian.  Charts  showing  the  normal  24-hour  movement  for  5° 
squares  have  been  prepared  to  take  the  place  of  the  earlier  charts  of 
paths  of  greatest  frequency.  Tables  covering  a  period  of  20  years 
are  also  available  for  the  average  velocity.  Thus  it  is  seen  that 
storms  of  continental  types  move  more  rapidly  in  winter  than  in 
summer. 


Kilome- 

Kilome- 

ters in  24 

Kites. 

ters  in  24 

Miles. 

hours. 

hours. 

January 

1  199 

745 

August 

788 

ffl 

February 

1  110 

690 

September 

883 

.549 

March    " 

1,080 

673 

October                       

919 

.-)71 

April 

871 

542 

November 

1,040 

646 

Mav  

792 

492 

December  

1,156 

718 

772 

480 

July....                                .  ... 

839 

521 

Average  for  year  

954 

593 

In  determining  a  possible  deviation  from  a  normal  course,  account 
is  taken  of  unequal  pressure  distribution  in  the  regions  adjacent  to 
the  storm  center,  also  the  location  of  maximum  12-hour  pressure 
fall  and  the  trend  of  the  isotherms.  A  number  of  empirical  rules 
based  upon  the  intensity  and  movement  of  the  12-hour  pressure 
change  are  given  by  Bowie  for  the  movement  of  lows. 

The  most  important  rules 3  for  the  guidance  of  the  forecaster  in 
determining  the  course  of  a  hurricane  are : 

A  hurricane  does  not  move  directly  toward  a  region  of  high  pressure  when  such  an 
area  is  not  moving  perceptibly,  but  follows  behind  it.  If  the  high  moves  east  or 
northeast  off  to  sea  at  a  normal  rate,  the  hurricane  moves  north  or  northeast.  If  the 

1  From  "Principles  of  Aerography,"  chapter  9,  A.  McAdie. 

2  Monthly  Weather  Review,  Suppl.  1,  July,  1914;  also  Suppl.  4,  Jan.,  1917. 

*  In  "Weather  Forecasting  in  the  United  States"  many  general  statements  are  made  by  the  various 
forecasters  regarding  the  movements  of  highs  and  lows. 

135 


136  MANUAL   OF  AEROGRAPHY. 

high  hangs  persistently  over  the  coast,  the  hurricane  is  deflected  far  to  the  west  before 
it  can  recurve. 

If  rain  falls  freely  before  the  hurricane  comes  to  land,  the  disturbance  may  decrease 
in  intensity;  but  if  heavy  rain  begins  after  the  storm  passes  inland,  the  storm  will 
probably  continue. 

When  a  West  Indian  hurricane  is  moving  westward  in  the  longitude  of  eastern  Cuba 
and  is  north  of  that  island,  it  will  recurve  east  of  the  South  Atlantic  coast,  when  an 
area  of  high  pressure  covers  the  Northwestern  States.  If  the  hurricane  is  moving 
westward  over  Cuba  or  the  western  Caribbean  Sea  when  a  low  area  occupies  the 
Northwest,  and  the  pressure  is  high  in  the  Eastern  States,  the  storm  will  probably 
move  to  the  Gulf  of  Mexico  and  reach  the  Gulf  coast  after  recurving. 

For  storms  over  the  Great  Lakes  it  appears  that  depressions  frequently  remain 
stationary  or  move  slowly,  accompanied  with  much  precipitation,  when  the  pressure 
is  high  to  the  north  and  northeast.  Again,  the  movement  is  slow  when  the  air  from 
an  extensive  high  pressure  area  drains  southeast  from  the  Missouri  Valley. 

Other  storms  that  increase  with  intensity  appear  to  depend  on  marked  horizontal 
temperature  gradients.  A  rapid  temperature  rise  in  front  of  a  storm  implies  an 
increase  in  intensity,  especially  if  the  temperature  is  falling  rapidly  over  the  North- 
west. Sharp  temperature  rises  in  the  eastern  quadrants  of  a  storm  are  a  sure  indi~ 
cation  that  the  storm  will  move  northeastward  and  increase  in  intensity. 

On  the  Atlantic  coast  there  are  certain  types  of  disturbance  which, 
especially  in  March,1  have  provoked  widespread  comment  owing  to 
the  failure  of  the  Washington  forecasters  rightly  to  anticipate  weather 
conditions  of  the  succeeding  24  hours.  Noteworthy  instances  were 
those  at  the  time  of  the  presidental  inaugurations  in  1897  and  1909. 

For  45  years  the  forecasters  at  Washington  and  for  shorter  periods 
at  other  forecast  centers  such  as  San  Francisco,  Chicago,  New  Orleans, 
Portland,  and  Denver  have  depended  mainly  in  making  their  fore- 
casts upon  certain  auxiliary  maps  of  pressure  and  temperature 
changes.  Attempts  have  been  made  to  use  cloud  change  and  humid- 
ity change  maps,  but  for  reasons  hardly  satisfactory,  it  would  seem, 
no  continuous  use  of  these  latter  charts  has  been  made.  The  pressure 
and  temperature  auxiliary  maps  show  the  24  and  12  hour  changes. 
The  barometric  tendency,  as  defined  by  the  International  Committee 
in  1913,  is  the  change  in  the  three  hours  preceding  the  observation. 
This  is  not  used  in  the  United  States,  but  whenever  the  pressure  has 
risen  or  fallen  1.02  mm  (1.5  kbj  within  two  hours  preceding  the 
observation,  the  change  is  reported,  though  not  the  character  of  the 
change,  such  as  steady,  unsteady,  etc. 

The  pressure  chart  gives  the  area  and  intensity  of  the  nonperiodic 
pressure  changes.  Henry,  compiling  the  fluctuations  in  pressure  at 
certain  stations  for  a  period  of  10  years,  found  that  the  frequency 
was  nearly  the  same  for  all  parts  of  the  country  except  that  the 
changes  are  more  rapid  at  northern  stations.  The  average  annual 
number  of  such  pressure  movements  is  88  and  the  average  time  inter- 
val four  and  two-tenths  days. 

1  Tho  snow  and  wind  storm  known  as  the  great  blizzard  of  New  York  occurred  March  12-14,  1888. 


MANUAL   OF    AEROGRAPH V.  137 

Ekholm  in  1911  suggested  the  terms  allohar  for  tlie  area  of  pressure 
change:  anallobar  for  an  area  over  which  the  pressure  has  risen;  and 
kat allohar  for  an  area  over  which  the  pressure  has  fallen  within  the 
given  time.  The  region  of  maximum  change  may  he  regarded  as 
the  center.  The  names  are  cumbersome  and  the  conditions  might 
well  be  described  simply  as  "rises"  and  "falls."  Henry1  has 
<lc-< Tihed  the  basis  of  forecasting  by  synoptic  maps,  and  given  at 
sonic  length  the  relation  of  the  pressure  change  areas  and  the  move- 
ments of  highs  and  lows.  Shaw  -  has  discussed  certain  relations 
between  the  isallobars  and  the  winds. 

Storms  over  the  Lakes  region  sometimes  develop  secondaries  off 
the  Virginia  or  New  Jersey  coasts;  and  these  pass  apparently  slowly 
northward,  causing  heavy  snows  and  high  winds  in  the  Middle 
Atlantic  and  Xew  England  States. 

Of  all  secondaries,  tornadoes  are  most  destructive  and  most  fre- 
quent. They  are  associated  with  storms  of  increasing  energy, 
moving  to  the  left  of  normal  paths  when  the  trough  of  low  pressure 
extends  well  southward.3  Again,  when  the  southern  portion  of  the 
trough  swings  eastward  faster  than  the  northern  portion,  there  is 
likelihood  of  the  development  of  a  secondary  storm  south  or  southeast 
of  the  northern  center.4 

There  is  a  tendency  for  secondaries  to  form  to  the  leeward  of  the 
Appalachian  Mountains,  following  the  passage  eastward  of  moderate 
disturbances  from  the  northwest.  A  pressure  rise  coming  from  the 
Lakes  region  seems  to  play  an  important  part.  If  this  moves  south 
of  the  low,  secondaries  do  not  develop. 

A.  McA. 

FORECASTING,  PART  2,  MOVEMENT  OF  LOWS.5 

There  are  ideas  which  are  of  importance  to  consider  in  the  daily 
forecast,  concurrently  and  simultaneously  with  the  isobaric  method. 
That  is,  the  union  of  the  two  methods  should  lead  sooner  or  later 
to  a  forecast  of  a  high  degree  of  perfection,  if  not  to  perfection  itself. 

We  will  imagine  ourselves,  in  a  central  station  at  the  time  when 
the  forecast  is  to  be  made,  before  an  isobaric  chart  of  Europe  which 
is  soon  to  be  covered  with  conventional  signs  and  multiple  isobars. 

The  sky  is  to  be  examined. 

Is  there  cirrus  ? 

If  yes,  we  will  apply  our  principle,  cirrus  comes  from  the  center  of 
the  depression,  and  the  importance  of  the  center  is  directly  propor- 

i  Weather  Forecasting  in  the  United  States,  p.  69. 

1  Forecasting  Weather,  pp.  337-341. 

>  Weather  Map  of  April  29,  1909. 

<  Weather  Map  of  November  8,  1913. 

*  Provision  du  Temps,  pp.  40-43.    (G.  Guilbert.) 


138  MANUAL   OF  AEBOGRAPHY. 

tional  to  the  speed  of  the  cirrus.  Rapid  cirrus,  strong  storm.  Slow 
cirrus,  weak  depression. 

Now,  it  is  necessary  to  first  determine  the  direction  and  speed  of 
the  cirrus.  Assume  the  northwest  (a  most  usual  direction)  and 
rapid  movement  (a  less  frequent  case).  We  may  deduce  from  that 
that  there  is  a  strong  storm  at  sea  off  the  northwest  advancing 
rapidly  toward  the  continent. 

Then  we  must  consider  the  force  and  the  direction  of  the  con- 
trolling winds  over  Europe,  principally  over  the  regions  menaced 
by  the  cirrus,  that  is  to  say,  by  the  new  storm  which  they  reveal. 

Two  principal  cases  may  present  themselves — either  the  winds  will 
be  convergent,  or  else  divergent,  always  in  relation  to  the  storm 
assumed  to  be  over  the  ocean. 

Thus,  with  the  depression  off  to  the  northwest,  convergence  of 
the  wind  is  perfect  if  the  wind  is  in  the  southwest.  It  is  then  an 
obstacle  to  the  advance  of  the  depression;  the  stronger  it  is,  the 
stronger  resistance  it  offers.  It  leads  the  air,  or  at  least  the  pressure, 
directly  toward  the  indicated  center,  from  southeast  to  northwest. 
It  is  important,  then,  to  consider  the  velocity  of  the  current  opposed 
to  the  storm  at  sea. 

Inversely,  instead  of  being  in  the  southwest,  the  wind  might  be 
in  the  north  or  northwest,  that  is,  in  the  same  direction  as  the  cirrus 
(a  frequent  case),  or  even  in  the  northeast. 

These  winds  are  all  divergent.  Far  from  opposing  the  least  resist- 
ance, they  favor,  on  the  contrary,  the  rapid  movement  of  the  distant 
depression.  They  constitute  for  it  a  center  of  call  or  attraction,  since 
the  movement  of  the  air,  which  takes  place  perpendicularly  to  these 
north  winds  from  west  to  east,  makes  a  hollow  in  front  of  it. 

It  is  then  essential  to  distinguish  immediatdly  between  the  con- 
vergence and  the  divergence  of  the  winds. 

It  is  next  necessary  to  examine  well  their  speed,  and  also  the  move- 
ment of  pressure  at  the  menaced  points. 

If  the  convergent  wind  is  weak,  and  the  drop  of  the  barometer 
has  already  made  itself  evident,  the  resistance  will  be  nil  and  of 
no  effect.  There  is  evident  disproportion  between  the  storm,  which  is 
very  powerful,  according  to  the  velocity  of  the  cirrus,  and  the  op- 
posing wind,  which  is  weak.  The  depression  will  advance,  then, 
with  its  initial  speed  toward  the  east,  according  to  the  general  laws 
'of  translation  of  storms. 

On  the  contrary,  if  the  convergent  wind  is  strong  or  violent,  with 
no  trace  of  a  barometric  drop  visible  in  the  most  advanced  stations, 
the  resistance  will  be  invincible,  and  might  even,  not  only  keep  the 
depression  at  sea,  but  push  it  back,  in  conformance  to  our  rule: 

Every  depression  which,  at  its  arrival  from  the  sea,  determines  abnormally  strong 
winds,  can  not  advance  and  will  remain  stationary,  if  it  is  not  even  thrown  back 
toward  its  place  of  origin. 


MANUAL  OF  AEROGRAPHY.  139 

For  example,  and  to  fix  the  idea,  suppose  that  a  1  mm.  (1.46  kb) 
drop  at  Valentia,  with  south  winds,  7  (very  strong),  is  due  to  a  strong 
storm  indicated  to  the  west  of  this  station  by  the  cirrus. 

There  is  every  evidence  that  there  is  disproportion  between  the 
effort  toward  a  lowering,  which  is  only  1  mm.  of  pressure,  and 
the  resisting  force,  which  is  represented  by  a  very  strong  convergent 
wind.  The  depression  will  be  forced  back;  the  barometer  will  rise 
again. 

Let  us  take  the  opposite  case.  The  barometer  is  down  10  mm  (14.6 
kb)  in  Ireland,  and.  the  wind  remains  weak  there,  2,  from  the  south. 
The  important  storm  revealed  by  the  clouds  will  meet  no  resistance, 
and  consequently  the  twisting  and  centrifugal  movement  will  develop 
still  more.  The  velocity  of  the  cyclone  being  considerable,  according 
to  the  cirrus,  the  atmospheric  perturbation  will  become  propor- 
tionately great,  and  will  provoke  a  vast  tempest. 

There  will  be  similar  condition  (imminent  danger)  if  the  winds, 
instead  of  being  convergent,  were  divergent.  In  this  case  even  with 
the  barometer  stiU  high  the  winds  will  be  preparing  a  powerful 
cyclone.  Far  from  piling  up  and  forming  a  dike  against  the  invasion 
of  the  storm,  far  from  constituting  thus  an  obstacle,  the  air  is  dis- 
persed in  front  of  the  advancing  center,  and  so  facilitates  its  rapid 
movement. 

The  more  violent  the  divergent  wind  is,  and  the  more  powerful 
the  attraction  of  the  air  toward  the  east,  and  the  greater  is  the  void 
established  in  front  of  the  storm,  by  so  much  will  the  initial  vortex 
be  accentuated,  and  a  tempest  is  inevitable. 

As  it  is  seen,  the  depression  at  sea,  invisible  on  the  isobaric  chart, 
may  be  indicated  by  the  observation  of  the  cirrus,  whose  velocity 
is  proportional  to  the  importance  of  the  center.  But  the  indication 
given  by  cirrus  will  only  be  justified  by  the  facts  if  the  surface  winds 
do  not  modify  the  initial  state  of  the  storm. 

The  strongest  storm  at  sea  may  be  indeed  broken  to  pieces  by  the 
excess  of  convergent  wind,  while  a  feeble  depression  may  be  trans- 
formed into  a  terrible  cyclone  by  very  powerful  divergent  winds. 

Forecasting  must  take  account  of  these  elementary  notions.  It 
will  be  so  much  the  surer  if  the  indications  given  by  the  clouds  be 
considered  with  the  data  of  the  isobaric  method.  In  every  case  our 
rules  permit  daily  appreciation  of  the  opposing  forces;  the  centrif- 
ugal and  the  centripetal  force  continually  in  battle  in  every  atmos- 
pheric situation. 


140  MANUAL   OF  AEROGKAPHY. 

FORECASTING,  PART  3,  TURNING  OF  WINDS  WITH  ALTITUDE.1 

With  a  distant  low  approaching  from  the  southwest,  surface  winds 
are  easterly  and  shallow,  and  above  them  is  a  layer  about  1  kilometer 
in  depth  in  which  there  is  little  or  no  wind;  above  this  layer  south- 
westerly winds  prevail. 

As  a  low  passes  north  of  the  station,  surface  winds  are  successively 
southeast,  south,  and  southwest,  and  the  turning  of  wind  with 
altitude  is  clockwise,  the  upper  winds  nearly  always  being  southwest 
to  west. 

With  a  low  northeast  of  the  station  and  a  high  southwest,  both 
surface  and  upper  winds  are  northwest.  As  this  high  approaches 
and  passes  soutl^of  the  station  the  surface  .winds  are  successively 
west-northwest,  west,  and  west-southwest,  turning  clockwise  with 
altitude  to  northwest. 

With  a  high  east  of  the  station  and  a  low  approaching  from  the  west 
or  west-northwest,  winds  are  southwest  and  strong  both  at  the 
surface  and  aloft. 

With  a  high  north  of  the  station  and  a  low  approaching  from  the 
southwest  and  passing  south  of  the  station,  surface  winds  are  north- 
northeast  to  east-northeast,  and  there  is  little  turning  up  to  4,000 
meters;  the  turning  at  higher  levels  is  counterclockwise  to  north- 
northwest  and  northwest. 

With  a  high  northwest  and  a  low  south  of  the  station,  surface  winds 
are  north  to  northeast,  turning  clockwise  with  altitude  to  northeast, 
and  at  higher  levels  counterclockwise  back  to  north-northwest. 

With  a  high  northwest  and  a  low  passing  northward  east  of  the 
station,  surface  winds  are  successively  north,  north-northwest,  and 
northwest,  turning  counterclockwise  with  altitude  to  northwest  and 
west-northwest. 

In  general,  the  turning  of  winds  with  altitude  is  usually  such  that 
they  have  a  westerly  component  before  the  3  km.  level  is  reached. 

1  Gregg,  W.  R.,  "Turning  of  winds  with  altitude."    (Monthly  Weather  Review,  January,  1918.) 


CHAPTER  XV 


SOME  METEOROLOGICAL  CONDITIONS  WHICH  INCREASE 
THE  DANGER  OF  FLYING 


141 


CHAPTER  XV. 


SOME    METEOROLOGICAL    CONDITIONS    WHICH    INCREASE 
THE  DANGER  OF  FLYING.1 

By  Capt.  C.  J.  P.  CAVE,  R.  E. 

It  may  seem  rajher  presumptuous  for  one  who  does  not  himself  fly 
to  discuss  the  dangers  that  may  be  met  with  in  the  air  as  though  a 
landsman  who  had  crossed  the  channel  a  few  times  were  to  write  on 
the  navigation  of  a  ship  across  the  ocean.  At  the  same  time,  it  may 
be  of  some  use  to  point  out  certain  conditions  of  the  atmosphere  which 
seem  to  me  to  constitute  dangers,  although  I  may  be  mistaken  in  my 
estimate  of  some  of  these,  and  would  welcome  any  information  from 
pilots  bearing  on  the  subject.  In  fact,  my  paper  is  meant  to  elicit 
information  rather  than  to  give  it. 

At  the  same  time  I  should  like  to  protest  very  strongly  against  the 
idea  that  we  have  made  so  much  progress  in  the  science  and  art  of 
aviation  that  we  can  afford  to  disregard  the  weather  altogether, 
except  perhaps  in  the  case  of  fog.  The  idea  is  a  common  one,  and  has 
been  often  stated,  but  it  seems  to  me  that  it  is  a  most  dangerous  idea 
to  foster.  An  airman  can  not  afford  to  disregard  the  weather  any 
more  than  can  a  seaman.  A  seaman  puts  to  sea  in  almost  any  weather, 
but  the  fact  that  storms  take  their  toll  of  shipping  is  a  proof  that 
seamen  can  not  entirely  disregard  these  things.  Neither  are  airmen 
immune.  Only  last  summer  it  was  stated  in  our  official  communique 
that  five  of  our  aeroplanes  had  failed  to  return  owing  to  a  severe  rain- 
storm. 

Probably  what  is  meant  when  it  is  stated  that  airman  can  afford  to 
disregard  the  weather  is  that  so  much  progress  has  been  made  in  the 
construction  and  design  of  aeroplanes  that  they  can  go  up  in  almost 
any  wind  and  can  fly  safely  in  winds  that  would  have  proved  fatal  to 
machines  a  few  years  ago.  But  there  remain  some  conditions  that 
are  dangerous,  and  many  that  are  severe  hindrances  to  aeroplane 
work  in  war. 

The  chief  conditions  that  suggest  themselves  to  my  mind  as  increas- 
ing the  risks  of  flying  are  the  following:  (1)  Gales;  (2)  squalls;  (3) 
bumps  and  eddies;  (4)  clouds;  (5)  rain,  hail  and  snow;  (6)  fog;  (7) 
lightning.  It  is  possible  that  the  number  might  be  added  to  by 

1  Reprinted  from  London  Aero  Journal,  Vol.  21,  p.  301,  1917. 

143 


144  MANUAL  OF   AEROGRAPHY. 

experienced  pilots,  and  it  is  also  possible  that  some  of  the  conditions 
that  seem  to  me  to  be  dangers  may  not  really  be  such,  but  I  suggest 
them  in  the  hope  of  getting  more  information. 

(1)  Gales. — In  the  early  days  of  flying  strong  winds  were  more 
formidable  than  to-day,  but  there  are  still  occasions  when  the  wind  is 
so  strong  that  machines  are  able  to  make  little  or  no  headway  against 
it,  and  such  strong  gale  may  arise  with  great  suddenness  and  some- 
times without  much  warning. 

An  example  occurred  on  December  28,  1914,  when  a  small  depres- 
sion formed  over  the  Bristol  Channel  and  passed  across  the  south  of 
England,  causing  a  gale  that  did  a  considerable  amount  of  damage. 
In  the  southeast  of  England  it  was  nearly  calm  before  the  onset  of 
the  gale,  which  sprung  up  with  great  suddenness/  At  Farnborough, 
for  instance,  an  anemometer  exposed  140  feet  above  the  ground 
level  registered  a  velocity  of  80  miles  per  hour,  when  only  a  quarter 
of  an  hour  previously  it  had  been  quite  calm.  I  do  not  know  that 
there  were  any  aeroplanes  flying  when  the  gale  began;  probably  not. 
The  gale  began  in  the  afternoon  in  the  southwest  of  England,  but  it 
did  not  reach  the  southeastern  counties  nor  the*  northeast  of  France 
till  after  dark.  But  a  gale  of  80  miles  per  hour  140  feet  above  the 
ground  must  have  been  considerably  more  at  a  height  of  1,500  feet, 
and  I  venture  to  think  that  if  aeroplanes  were  flying  when  such  a 
wind  sprang  up  many  would  have  failed  to  return  to  their  aerodromes. 
If  at  the  same  time  there  were  a  development  of  low  clouds  when  such 
a  gale  came  on,  aeroplanes  would  be  likely  to  lose  their  bearings 
entirely. 

Besides  the  loss  of  aeroplanes  actually  flying,  a  gale  such  as  the  one 
under  discussion  would  cause  severe  damage  to  hangars  and  tents, 
and  I  believe  that  on  this  occasion  considerable  damage  was  caused 
in  this  way.  Anyone  who  was  in  northeastern  France  at  the  time 
may  remember  the  tiles  and  chimney  pots  with  which  the  roads 
through  villages  were  strewn. 

Gales  may  spring  up  with  great  suddenness  at  all  times  of  the  year; 
in  fact,  I  think  that  a  summer  gale  may  often  give  less  warning  of 
its  approach  than  a  winter  one.  Anyone  who  has  done  any  sailing 
round  our  coasts  must  remember  cases  when  they  have  been  caught 
in  gales  a  couple  of  hours  or  so  after  having  been  lying  becalmed.  I 
think  that  easterly  gales  in  the  summer  are  apt  to  spring  up  suddenly 
in  this  way. 

A  gale  however,  usually  gives  warning  of  its  approach,  and  may 
often  be  forecasted  many  hours  in  advance,  and  the  weather  map 
for  the  day  will  usually  indicate  when  a  gale  is  likely. 

(2)  Squalls. — A  squall  is  a  temporary  rise  in  the  wind  above  the 
mean  velocity  that  preceeds  and  follows  it,  the  rise  in  velocity  being 
continued  over  some  minutes  at  least,  and  is  thus  distinguished  from 


MANUAL  OF   AEROGRAPHY.  145 

a  gust,  which  only  lasts  a  small  part  of  a  minute.  Squalls  are  of 
innumerable  degrees  of  severity.  On  a  day  of  blustery  northwest 
winds,  when  there  are  large  cumulus  clouds  about,  one  may  have  a 
succession  of  squalls,  whose  approach  can  be  seen  at  sea  some  time 
before  their  onset.  Such  squalls  are  probably  not  of  any  particular 
danger  to  aeroplanes,  as  at  sea  they  are  of  not  much  danger  to  ship- 
ning,  except  in  the  case  of  small  open  sailing  boats,  but  in  peace  time 
at  seaside  resorts  they  take  their  toll  of  holiday  makers  who  are  sailing 
with  the  main  sheet  made  fast. 

More  intense  squalls  are  associated  with  thunderstorms,  and  they 
are  all  the  more  dangerous  since  they  are  often  preceded  by  very 
light  winds  or  even  by  a  complete  calm,  and  within  a  minute  or  so 
from  their  onset  the  wind  is  blowing  at  the  rate  of  even  60  to  80 
miles  per  hour.  A  typical  example  of  such  a  squall  occurred  on 
August  2,  1906.  As  seen  in  the  East  of  Hampshire,  this  storm  came 
up  from  the  direction  of  the  Isle  of  Wight  in  the  shape  of  a  huge 
cumulus  cloud  with  a  great  extension  of  false  cirrus  .at  the  top, 
giving  it  the  appearance  of  a  giant  mushroom;  the  day  had  been 
very  hot  and  the  air  was  very  still.  As  the  storm  approached  it 
was  seen  that  heavy  rain  was  falling,  but  there  was  no  sign  of  wind 
to  the  untrained  eye.  A  few  minutes  before  the  rain  reached  the 
observer  a  continuous  roar  was  heard,  and  as  the  first  drops  fell  a 
furious  blast  of  wind  arose;  the  wind  only  lasted  a  few  minutes, 
and  in  three-quarters  of  an  hour  the  storm  had  passed  and  the 
weather  was  fine  again.  The  storm  passed  over  the  South  Downs, 
and  the  same  storm  or  another  moving  parallel  to  it  reached  Guild- 
ford,  where  the  damage  done  by  the  wind  was  very  great.  I  can 
not  imagine  that  an  aeroplane  caught  in  such  a  squall  would  not 
have  been  in  danger.  Certainly  an  airship  would  have  been  in  the 
very  greatest  danger  and  could  hardly  have  weathered  the  storm. 

The  squall  in  front  of  an  ordinary  thunderstorm  is  probably  a 
modification  of  another  variety  known  as  the  line  squall.  The 
sequence  of  events  in  a  line  squall  is  somewhat  as  follows:  A  bank 
of  clouds  is  seen  extending  along  the  horizon,  the  upper  parts  being 
white  and  in  shape  like  ordinary  cumulus,  though  the  whole  cloud 
usually  appears  of  a  more  uniform  height,  and  not  broken  up  into 
such  distinct  peaks  as  is  ordinary  cumulus.  As  the  cloud  approaches 
it  is  seen  to  be  extremely  dark  below,  and  it  usually  extends  in  a 
long  line,  stretching  from  horizon  to  horizon,  but  owing  to  the 
effect  of  perspective  it  appears  like  an  arch  in  the  sky.  the  summit 
of  the  arch  coming  nearer  and  nearer  overhead.  As  the  cloud 
reaches  the  observer  a  violent  squall  springs  up.  the  wind  veers 
rapidly  or  even  suddenly,  rain  falls  in  torrents  and  is  often  accom- 
panied by  hail,  and  there  may  be  thunder  and  lightning;  at  the 
same  time  the  temperature  falls  considerably,  a  fall  of  five  or  ten 
50821—18—10 


146  MANUAL  OF   AEEOGRAPHY. 

degrees  being  common,  and  it  is  sometimes  as  much  as  20  degrees. 
When  the  cloud  is  approaching  and  is  nearly  overhead  a  curious 
sinuous  line  is  seen  at  its  base  extending  right  along  the  front  of  the 
cloud,  and  it  is  this  line  which  gives  the  name  of  line  squall  to  the 
disturbance.  After  the  first  blast  the  wind  blows  strongly  for  a 
time  and  the  heavy  rain  lasts  for  half  an  hour,  more  or  less;  this  is 
followed  by  a  less  intense  fall  of  rain  and  by  decreasing  wind,  and 
often  in  an  hour  or  so  the  weather  clears  up  and  becomes  fine. 

A  line  squall  is  only  a  few  miles  across,  but  it  may  be  several  hundred 
miles  long,  and  it  advances  across  the  country  broadside  on  at  the 
rate  of  20  to  40  miles  per  hour;  one  such  squall  has  been  traced  from 
Cape  Wrath  to  the  center  of  France,  another  from  the  Northwest 
of  Ireland  to  the  center  of  Germany.  The  list  of  disasters  caused  by 
line  squalls  is  a  long  one;  the  best  known  case  is  that  of  H.  M.  S. 
Eurydice,  a  training  ship  homeward  bound  that  was  struck  by  such  a 
squall  when  off  the  Isle  of  Wight  on  March  24,  1878,  and  foundered 
with  heavy  loss  of  life. 

Besides  the  blast  of  wind  in  front  of  the  squall,  there  are  great  up 
currents  in  front  and  down  currents  near  the  middle  of  the  squall, 
with  much  eddy  motion  between  them.  Such  conditions  could 
hardly  fail  to  be  dangerous,  and  though  an  aeroplane  might  possibly 
come  safely  through  them,  it  is  hardly  likely  that  an  airship  would. 

The  onset  of  a  line  squall  is  generally  sudden,  though  anyone  with 
a  very  little  training  in  meteorology  can  see  it  coming  while  it  is  still 
some  way  off.  On  January  20,  1916,  a  line  squall  passed  across  the 
country  from  northwest  to  southeast,  reaching  Farnborough  at  10.30 
a.  m.  The  morning  had  been  fine,  and  a  number  of  machines  were 
out  on  the  common;  the  squall  came  on  suddenly,  and  several 
machines  were  damaged  before  they  could  get  back  into  their  sheds. 
A  storm  such  as  this  one  can  be  predicted  with  some  success  if  the 
machinery  is  ready  for  the  purpose.  The  general  conditions  favor- 
able for  line  squalls  can  usually  be  forecasted  from  the  Daily  Weather 
Map  prepared  at  the  Meteorological  Office,  but  unless  a  line  squall 
occurred  at  one  of  the  Meteorological  Office  Observing  Stations  at  the 
time  of  taking  of  the  meteorological  observations  or  shortly  before, 
the  existence  of  the  squall  may  not  be  noticed  on  the  map.  The 
squall  of  January  20,  1916,  was  first  observed  in  the  south  of  Ireland 
at  about  4.30  a.  m.  It  was  accompanied  by  much  thunder  and  light- 
ning. It  crossed  the  Irish  Sea  and  reached  the  coasts  of  Cornwall 
and  Wales  at  about  7.30  a.  m.,  and  moved  across  the  country  in 
a  line  100  miles  long  or  so,  the  movement  being,  as  usual,  at  right 
angles  to  its  length.  Now,  this  storm  did  not  affect  any  of  the 
Meteorological  Office  Observing  Stations,  and  hence  its  existence 
was  not  officially  known,  as  one  may  express  it.  But  since  a  squall 
of  this  type  is  perfectly  easily  recognized,  its  coming  might  have 


XI-AI.    OF    A!,!;<M,|;A|'HY.  147 

been  foretold  if  the  proper  machinery  liad  heen  in  existence  to  deal 
with  it;  such  a  warning  might  have  hern  received  in  plenty  of  time 
for  the  aeroplanes  on  the  common  a!  Karnborough  to  have  heen  put 
in  their  sheds  in  safety. 

That  there  is  time  for  such  warnings  to  he  given  is  shown  by  the 
fact  that  recently  I  had  a  telegram  from  Tpavon  telling  me  that  a 
line  squall  had  just  passed  over:  the  telegram  wa>  received  about 
10  minutes  before  the  squall  reached  Farnborough.  Ten  minutes 
i<  doubtless  too  >hort  a.  time  for  the  warning  to  be  acted  upon,  but 
a  station  further  west  than  I'pavon  could  have  sent  a,  warning  that 
would  have  heen  received  in  plenty  of  time. 

It  appears  to  me  that  it  would  be  quite  feasible  for  an  observer  at 
every  aerodrome  to  send  a  telegram  to  some  central  office  when  a 
line  squall  took  place:  at  the  central  office  the  general  weather 
conditions  would  be  well  known,  and  therefore  the  direction  of 
motion  of  the  squall  could  be  foretold.  In  addition,  other  reports 
would  come  in  as  the  squall  reached  other  aerodromes,  and  the  rate 
of  travel  could  be  obtained  with  some  accuracy.  Warnings  could 
_then  be  sent  to  all  aerodromes  which  are  likely  to  be  affected,  and 
some  signal  might  be  hoisted  in  a  conspicuous  place  where  it  could 
be  -ecu  not  only  by  those  who  were  responsible  for  machines  on  the 
ground,  but  also  by  pilots  who  were  flying  in  the  immediate  neigh- 
borhood of  the  aerodrome.  Some  such  organization  would  not  be 
complicated,  and  would  only  occasionally  have  to  be  put  into  use, 
but  it  might  be  the  means  of  saving  machines,  and  possibly  lives 
also.  If  the  information  were  to  be  sent  out  by  wireless  the  warn- 
ings would  be  received  still  earlier.  But  weather  forecasters  are 
looked  on  in  some  quarters  as  the  subjects  for  jokes,  and  it  will  take 
some  serious  accidents  caused  by  line  squalls  before  anything  prac- 
tical is  likely  to  be  done.  A  severe  line  squall  is  generally  accom- 
panied by  thunder  and  lightning,  and  an  automatic  lightning 
recorder  would  indicate  its  approach,  especially  in  winter,  when 
thunderstorms  are  almost  entirely  of  the  line-squall  type.  The 
lightning  recorder  at  Farnborough  began  to  record  lightning  at 
about  5  a.  m.  on  January  20,  1916,  and  it  is  quite  evident  from  the 
chart  that  something  quite  out  of  the  ordinary,  for  the  winter,  was 
occurring. 

(3)  Bumps  and  eddies.— I  do  not  propose  to  deal  with  these  at 
any  length.  The  danger  from  bumps  is  small  with  modern  aero- 
planes, though  in  the  early  days  of  flying  they  were  a  source  of  great 
danger.  Pilots  are  far  better  qualified  to  speak  of  bumps  than  is  a 
meteorologist  who  has  only  flown  a  few  times  as  a  passenger.  Bumps 
are  mostly  due  to  rising  currents  of  air  over  surfaces  of  ground  that 
are  at  different  temperatures  and  to  eddy  motion  due  to  the  wind 
blowing  over  irregularities  of  the  surface.  They  also  seem  to  occur 


148  .         MANUAL  OF   AEKOGRAPHY. 

at  the  cloud  layer  when  there  is  a  sheet  of  cloud,  and  they  occur,  of 
course,  with  cumulus  clouds. 

In  this  connection  there  is  a  point  on  which  pilots  could  give  some 
information.  At  Farnborough  an  easterly  wind  is  far  more  bumpy 
than  a  westerly  wind.  Is  this  due  to  some  local  configuration  of  the 
ground,  or  is  it  something  inherent  in  an  easterly  wind?  I  suspect 
that  there  is  some  connection  between  bumpiness  and  easterly  winds, 
but  what  it  is  I  can  not  attempt  to  explain.  An  easterly  wind  has, 
seemingly,  peculiarities  of  its  own;  it  is  said,  for  instance,  that  there 
is  always  more  sea  in  the  channel  with  an  easterly  wind  than  with 
a  wind  of  corresponding  strength  from  other  quarters. 

(4)  Clouds. — Clouds  may  be  a  danger  in  several  ways.  In  the 
case  of  cumulo  nimbus  clouds  the  heavy  rain,  and  possibly  hail,  or 
the  snow  in  winter,  may  prove  extremely  dangerous.  Such  clouds, 
too,  are  often  of  great  extent  and  are  the  seat  of  very  rapidly  ascend- 
ing currents  of  air.  A  pilot  might  have  to  fly  a  considerable  distance 
before  getting  clear  and  might  easily  lose  his  bearings.  A  cumulo 
nimbus  cloud  is  one  that  should  be  avoided  if  it  is  possible  to  do  so, 
and  as  these  clouds  often  occur  in  isolated  masses,  it  may  be  at  times 
possible  to  fly  around  them.  A  cumulo  nimbus,  the  true  shower 
cloud,  from  which  rain,  hail,  or  snow  is  falling  may  be  distinguished 
from  simple  cumulus  by  the  fact  that  the  top  of  the  former  cloud, 
instead  of  being  rounded  and  hard  edged,  is  brushed  out  into  a  soft- 
looking  mass  of  fibrous  cloud  called  false  cirrus.  It  is  true  that 
showers  fall  from  simple  cumulus  clouds,  but  the  really  heavy  falls 
are  from  cumulo  nimbus. 

Cumulus  clouds  are  more  common  in  summer  than  in  winter. 
They  usually  begin  to  form  in  the  morning  as  the  day  gets  warm,  and 
reach  their  greatest  development  about  2  or  3  in  the  afternoon, 
after  which  they  generally  begin  to  disappear,  and  by  sunset  or  soon 
after,  the  sky  may  be  quite  clear  even  after  a  day  of  great  develop- 
ment of  this  form  of  cloud.  But  they  may  be  met  with  at  other 
times,  and  they  form  after  sunset  in  the  summer  when  shallow 
depressions,  bringing  thunderstorms,  are  approaching. 

Low  sheets  of  cloud  may  prove  a  hindrance  to  work  with  aeroplanes, 
and  if  they  are  very  low  they  may  cause  difficulties  for  a  pilot  in 
finding  his  way  back  to  the  aerodrome,  or  even  difficulties  in  landing 
at  all.  The  possibility  of  low  sheets  of  cloud  forming  after  a  clear 
morning,  especially  in  winter  during  unsettled  weather,  should 
always  be  borne  in  mind.  A  glance  at  the  latest  weather  map  will 
often  be  a  valuable  guide  as  to  whether  a  fine  morning  is  likely  to 
last. 

Take,  for  instance,  the  map  for  7  a.  m.  on  December  24  last.  This 
shows  that  the  sky  was  clear  in  northeastern  France,  but  subsequent 
reports  show  that  it  became  overcast  and  rainy,  and  that  there  was  a 


MANUAL  OF   AEROGRAPHY.  149 

great  extension  of  low  clouds.  Anyone  who  had  in  the  least  followed 
the  weather  in  the  days  preceding  this  date  and  who  had  seen  the  map 
for  the  day,  or  even  for  the  preeedin^  evening,  would  have  realized 
that  the  fine  morning  was  not  likely  to  last  beyond  a  short  time.  I 
maintain  that  aeroplanes  going  up  on  such  a  morning  would  be 
extremely  likely  to  find  low  clouds  extending  to  within  a  few  hundred 
feet  of  the  ground  before  their  return,  and  they  would  therefore  be 
in  some  danger.  It  might  be  that  it  would  be  necessary  to  incur 
the  danger  for  the  results  that  might  be  obtained,  but  it  would  be 
absurd  to  say  that  such  meteorological  conditions  would  not  add  to 
the  risks  of  flying. 

(5)  Rain,  hail,  and  snow. — The  danger  arising  from  these  is 
obvious,  and  it  varies,  of  course,  from  nothing  in  the  case  of  very 
light  falls  to  a  real  danger  hi  the  case  of  very  heavy  falls.  Danger 
from  very  heavy  rain  or  from  hail  can  often  be  avoided  by  keeping 
away  from  cumulo  nimbus  clouds.  Such  clouds  are  often  seen  in 
isolated  masses,  some  miles  in  circumference,  perhaps,  but  still  they 
can  on  such  occasions  be  avoided.  In  the  case  of  a  line  squall  it  is, 
however,  not  possible  to  fly  round  the  cloud,  for  this  often  extends  in 
a  long  line  for  several  hundred  miles.  It  might  be  possible  to 
fly  over  a  line  squall  and  so  avoid  the  rain,  hail,  and  turbulent 
motion  of  the  air  associated  with  the  disturbance,  but  I  do  not  know 
that  there  is  any  evidence  as  to  the  heights  to  which  the  line  squall 
disturbance  extends;  it  probably  varies  on  different  occasions. 

Hailstones  in  summer  thunderstorms  constitute  a  real  danger  to  an 
aeroplane  that  might  be  involved  in  the  storm,  for  they  sometimes 
attain  an  enormous  size.  In  the  British  Rainfall  Organization's 
Volume  for  1913  is  a  photograph  of  hailstones  that  fell  in  Essex  on 
May  27  of  that  year.  Although  they  had  partially  melted  before 
they  were  photographed,  they  are  still  shown  as  nearly  as  large  as  a 
hen's  egg  that  was  photographed  with  them  for  comparison.  They 
fell  at  Wickham  St.  Pauls,  near  Halsted,  in  Essex.  At  Great  Yeld- 
ham  there  was  a  fall  of  similar  hailstones.  At  the  latter  place  the  fall 
lasted  only  eight  minutes,  but  "the  devastation  was  great,"  an 
observer  says.  " Crops  were  smitten  to  the  ground,  glasshouses  all 
smashed,  tarred  roof  felting  cut  to  ribbons,  corrugated  iron  riddled, 
tiles  and  windows  smashed  in  thousands.  *  *  *  Man  and  beast 
were  bruised,  and  among  other  animals  killed  on  my  own  farm  I  saw 
rooks,  wood  pigeons,  full-grown  hares,  partridges,  pheasants,  rabbits, 
and  various  small  birds,  wild  ducks,  farmyard  fowls,  and  three 
cygnets."  At  Haverhill  the  stones  varied  from  the  size  of  nutmegs 
to  that  of  walnuts.  At  Harston  the  stones  were  25  millimeters  to  32 
millimeters  (1  to  1£  inches)  in  diameter,  and  at  Sheerness  16  milli- 
meters to  22  millimeters  (f  to  J  inch).  Damage  from  hail  was  re- 
ported from  a  wide  region  in  the  eastern  counties  on  that  day. 


150  MANUAL   OF    AEROGRAPH Y. 

Doubtless  it  will  be  said  that  this  was  an  exceptional  occasion, 
and  no  doubt  it  was,  but  a  glance  through  one  of  the  rainfall  vol- 
umes shows  that  very  heavy  falls  of  hail  occur  in  the  summer  months 
in  all  parts  of  England.  For  instance,  besides  the  24th  of  the  month, 
heavy  falls  occurred  in  May,  1913,  on  several  occasions;  on  the  19th 
very  heavy  rain  fell  at  Bolerno,  in  Midlothian;  on  the  26th  hailstones 
over  1  inch  in  diameter  fell  at  Bulvan,  and  at  Bishops  Castle  scores 
of  windows  were  shattered;  on  the  30th  at  Gravesend  the  hailstones 
were  1^  inches  in  diameter;  on  the  31st  at  Waltham,  on  the  Wolds, 
there  was  an  exceptionally  heavy  fall  of  hail. 

A  glance  through  the  records  for  any  summer  month  shows  that 
so-called  exceptional  falls  of  hail  are  fairly  common,  and  it  scarcely 
needs  pointing  out  that  hailstones  far  smaller  than  hen's  eggs  would 
have  fatal  effects  on  an  aeroplane  that  met  them. 

(6)  Fog. — Perhaps  fog  is  one  of  the  worst  of  the  dangers  that  beset 
flying,  and  I  should  like  especially  to  call  attention  to  fog  to  those 
who  maintain  that  at  the  present  time  aviators  can  afford  to  dis- 
regard the  weather.     The  subject  of  fog,  however,  has  lately  been 
dealt  with  before   this   society  by  Maj.  Taylor.     I   may,  however, 
give  an  example  of  howT  a  fog  may  be  formed  by  the  mixing  of  air 
at  different  temperatures.     On  April  3  of  this  year  there  was   a 
shower  of  snow  at  Farnborough  which  was  followed  immediately  by 
bright  sunshine;  after  a  few  minutes  a  mist  began  to  form  over  the 
aerodrome,  and  for  a  short  time  wreaths  of  fog  covered  the  common. 
No  doubt  the  sun  heated  the  ground  and  warmed  the*  air  in  contact 
with  it;  the  relatively  warm  air,  which  would  have  been  fully  saturated 
was  mixed  with  the  cold  layer  just  above — air  that  had  been  cooled 
by  the  melting  snow  before  the  sun  came  out.     The  saturated  warm 
air  was  chilled  and  its  moisture  was  condensed  into  fog. 

(7)  Lightning. — It  is  difficult  to  say  what  is  the  danger  to  be 
apprehended  from  lightning  as  such.     The  dangers  to  flying  from 
thunderstorms  are  due  to  the  squalls  and  to  the  heavy  rain  and  hail 
that  accompany  them.     It  is  possible  that  the  actual  danger  from 
lightning  to  an  aeroplane  flying  through  a  thunderstorm  may  be  no 
more  than  that  incurred  by  a  pedestrian  walking  across  an  open 
common  during  a  storm.     A  pilot  who  was  flying  above  a  thunder- 
cloud last  summer  reported  that  long  sparks  were  given  off  by  his 
machine  at  intervals.     It  is  very  likely  that  this  happened  every 
time  there  was  a  flash  of  lightning  from  the  cloud  below  him.     But  no 
inconvenience  was  caused  by  the  sparks.     In  the  case  of  an  airship, 
however,  it  would  be  far  otherwise,  for  quite  small  sparks  might 
ignite  the  hydrogen.     Lightning  is  also  a  danger  to  kite  balloons 
owing  to  the  conducting  wire.     There  are  several  cases  on  record  in 
which  meteorological  kites  have  been  struck  by  lightning,  and  as 
some  of  these  occurred  when  there  was  no  thunderstorms  in  progress > 


MANUAL    OF    AKROGRAPHV.  151 

it  must  be  remembered  that  the  clouds  may  be  highly  charged  with 
electricity  at  times  when  no  actual  storm  is  going  on. 

There  is  a  particular  type  of  violent  thunderstorm  in  which  most 
of  the  lightning  takes  place  from  rloud  to  cloud,  and  when  it  is  almost 
incessant.  It  would  probably  be  dangerous  for  an  aeroplane  to  enter 
such  a  cloud,  but  the  appearance  of  the  danger  is  so  obvious  that  it 
scarcely  needs  pointing  out. 

I  hope  that  what  I  have  said  will  make  it  clear  that  the  airman  is 
not  entirely  immune  from  the  disturbances  in  the  medium  in  which 
he  flies.  If  I  could  persuade  aviators  in  general  to  take  meteorology 
rather  more  seriously  I  should  feel  that  I  had  not  read  this  paper  in 
vain,  but  I  am  afraid  that  weather  maps  and  forecasts  are  looked  on  by 
many,  though  not  by  all,  I  am  glad  to  think,  as  mere  matters  of  guess- 
work, and  not  worthy  of  serious  consideration.  The  public  in  general 
exhibit  a  lamentable  ignorance  of  the  very  elements  of  meteorology, 
which  is  largely  due  to  our  educational  methods.  I  do  not  wish 
everyone  to  become  a  meteorologist,  but  there  is  no  reason  why 
everyone  should  not  take  an  intelligent  interest  in  the  movements  of 
depressions  and  anticyclones,  and  have  some  faint  knowledge  of  what 
these  terms  mean.  The  English  are  supposed  to  talk  so  much  about 
the  weather  that  it  is  a  pity  they  should  not  know  what  they  are 
talking  about,  and  those  who  are  responsible  for  the  safety  of  aero- 
planes and  airships  ought  to  know  as  much  about  the  weather  as, 
say,  a  master  mariner  in  the  mercantile  marine.  There  are  cases 
where  ignorance  may  be  criminal. 

r.  j.  P.  c. 


CHAPTER  XVI. 


TABLES. 


153 


CHAFfER  XVI. 


TABLE  1. — Inches  into  millimeters. 
[1  inoh=25.40005  millimeters.] 


Inches. 

0.00 

0.01 

0.02 

0.03 

0.04 

0.05 

0.06 

0.07 

0.08 

0.09 

0.00. 

Mm. 
0.00 

Mm. 
0.25 

Mm. 
0.51 

Mm. 
0.76 

Mm. 
1.02 

Mm. 
1.27 

J0». 
1.52 

Mm. 
1.78 

Mm. 
2.03 

Mm. 
2.29 

(i  10 

2.54 

2.79 

3.05 

3.30 

3.56 

3.81 

4.06 

4.32 

4.57 

4.83 

0  20 

5  08 

5  33 

5  59 

5  84 

6  10 

6  35 

6  60 

6  86 

7  11 

7  37 

0.30. 

7.62 

7.87 

8.13 

8.38 

8.64 

8.89 

9.14 

9.40 

9.65 

9.91 

0  40 

10.16 

10.41 

10.67 

10.92 

11.18 

11.43 

11.68 

11.94 

12.19 

12  45 

0.50. 

12.70 

12.95 

13.21 

13.46 

13.72 

13.97 

14.22 

14.48 

14.73 

14.99 

0  60 

15  24 

15.49 

15.75 

16.00 

16.26 

16.51 

16.76 

17.02 

17.27 

17  53 

II.  7d  

17.7s 

18.03 

18.29 

18.54 

18.80 

19.05 

19.30 

19.56 

19.81 

20.07 

0.80 

20.32 

20.57 

20.83 

21.08 

21.34 

21.59 

21.84 

22.10 

22.35 

22.61 

0  90 

22.86 

23.11 

23.37 

23.62 

23.88 

24.13 

24.38 

24.64 

24.89 

25  15 

1  00 

25  40 

TABLE  2. — Feet  into  meters. 

[1  foot =0.3048006  meter.] 


Feet. 

0 

1 

2 

3 

4 

5 

6 

• 

8 

9 

0 

Meters. 
0.000 

Mft,  r,. 

0.305 

Meters. 
0.610 

Meters. 
0.914 

Meters. 
1.219 

Meters. 
1  524 

Meters. 
1  829 

Meters. 
2.134 

Meters. 
2  438 

Meters. 
2  743 

»::: 

3.048 

3.353 

3.658 

3.962 

4.267 

4.572 

4.877 

5.182 

5.486 

5.791 

20 

6.096 

6.401 

6.706 

7.010 

7.315 

7.620 

7.925 

8.230 

8.534 

8  839 

30  " 

9  144 

9  449 

9  754 

10  058 

10  363 

10  668 

10  973 

11  278 

11  582 

11  887 

40 

12.  192 

12.  497 

12.802 

13.106 

13.  411 

13.  716 

14.021 

14.326 

14.630 

14  935 

50 

15  240 

15  545 

15  850 

16  154 

16  459 

16  764 

17  069 

17  374 

17  678 

17  983 

i» 

Is  .NX 

18.593 

Is   s'.ls 

19.202 

19.507 

19  812 

20.117 

20.422 

20.726 

21  031 

70 

21  336 

21  641 

21  946 

22  250 

22  555 

22  still 

23  165 

23  470 

23  774 

24.079 

so:: 

24.384 

24.689 

24.994 

25.298 

2:>  I'm:', 

2.')  '.His 

26.213 

26.518 

2f.  s22 

27  127 

90 

27.  432 

27  737 

28  042 

28.346 

28.651 

28  956 

29  261 

29  566 

29  870 

30  175 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100... 

30.48 

33.53 

36.58 

39.62 

42.67 

45  72 

48  77 

51  82 

54  86 

57  91 

200 

60  96 

64  01 

67  06 

70  10 

73  15 

76  20 

79  25 

82  30 

85  34 

88  39 

300  

91  44 

94  49 

97  54 

100  58 

103  63 

106  68 

109  73 

112  78 

115  82 

118.87 

400 

121  92 

124  97 

128  02 

131  06 

134  11 

137  16 

140  21 

143  26 

146  30 

149  35 

500.... 

152  40 

155  45 

158  50 

161  54 

164  59 

167  64 

170  69 

173  74 

176  78 

179  83 

600... 

182  88 

185  93 

188  98 

192  02 

195  07 

198  12 

201  17 

204  22 

207  26 

210  31 

700 

213  36 

216  41 

219  46 

222  50 

225  55 

228  60 

231  65 

234  70 

237  74 

240  79 

800 

243  g4 

•'I1'    VI 

•>4>)  i)t 

•}-,•>    i|S 

250  03 

•>-.!)     |)y' 

262  13 

2fi.T     18 

2<i8  22 

271  27 

900. 

274  32 

277  37 

280  42 

•'x.H  4i', 

''Sl'i    ~)\ 

•>VJ      )fi 

292  61 

•7<|-,  i;c> 

298.70 

301  75 

1,000  

304.80 

TABLE  3.— Miles  into  kilometers. 
[1  mile=l. 609347  kilometers.] 


Miles. 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

0... 

Kilo- 
meters. 

o 

Kilo- 
meters. 
2 

Kilo- 
meters. 
3 

Kilo- 
meters. 
5 

Kilo- 
meters. 
6 

Kilo- 
meters. 
3 

Kilo- 
meters. 
10 

Kilo- 
meters. 
11 

Kilo- 
meters. 
13 

Kilo- 
meters. 
14 

10. 

16 

18 

19 

21 

23 

24 

26 

27 

29 

31 

20 

32 

34 

35 

37 

QQ 

40 

42 

43 

45 

47 

30. 

48 

50 

51 

53 

55 

56 

58 

60 

61 

63 

40 

64 

66 

68 

69 

-} 

72 

74 

76 

77 

79 

50. 

80 

82 

84 

85 

87 

89 

90 

92 

93 

95 

60 

97 

98 

100 

101 

103 

105 

106 

108 

109 

111 

70. 

113 

114 

116 

117 

119 

121 

122 

124 

126 

127 

H) 

129 

130 

132 

134 

135 

137 

138 

140 

142 

143 

90  

145 

146 

148 

150 

151 

153 

154 

156 

158 

159 

100  

161 

155 


156 


MANUAL  OF   AEKOGRAPHY. 


TABLE  4. — Kilometers  into  miles. 
[1  kilometer=0.621370  mile.] 


Kilometers. 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

0 

Miles. 
0.0 

Miles. 
0.6 

Miles. 
1.2 

Miles. 
1.9 

Miles. 
2.5 

Miles. 
3.1 

Miles. 
3.7 

Miles. 
4.3 

Miles. 
5.0 

Miles. 
5.6 

l6::: 

6.2 

6.8 

7.5 

8.1 

8.7 

9.3 

9.9 

10.6 

11.2 

11.8 

20  

30 

12.4 
18.6 

13.0 
19.3 

13.7 
19.9 

14.3 
20.5 

14.9 
21.1 

15.5 
21.7 

16.2 
22.4 

16.8 
23.0 

17.4 
23.6 

18.0 
24.2 

40  

24.9 

25.5 

26.1 

26.7 

27.3 

28.0 

28.6 

29.2 

29.8 

30.4 

50 

31.1 

31.7 

32.3 

32.9 

33.6 

34.2 

34.8 

35.4 

36.0 

36.7 

60  

37.3 

37.9 

38.5 

39.1 

39.8 

40.4 

41.0 

41.6 

42.3 

42.9 

70. 

43.5 

44.1 

44.7 

45.4 

46.0 

46.6 

47.2 

47.8 

48.5 

49.1 

80 

49  7 

50  3 

51.0 

51  6 

52.2 

52.8 

53.4 

54.1 

54.7 

55.3 

90. 

55.9 

56.5 

57.2 

57.8 

58.4 

59.0 

59.7 

60.3 

60.9 

61.5 

100 

62  1 



TABLE  5. — Conversion  of  area,  volume,  weight,  and  pressure. 


Multiply— 

By- 

To  convert  to— 

Miles 

1  6093 

Kilometers 

Kilometers  . 

.  62137 

Miles. 

Square  inches  

6.  4517 

Sqare  centimeters. 

Square  centimeters  

.155 

Square  inches. 

Square  feet 

092903 

Square  meters. 

Square  meters  

10.  7639 

Square  feet. 

Cubic  inches 

10.  387 

Cubic  centimeters.- 

Cubic  centimeters  

.  061025 

Cubic  inches. 

Cubic  feet  

.  028317 

Cubic  meters. 

Cubic  meters 

35  314 

Cubic  feet 

Cubic  feet  

28.  317 

Liters. 

Liters 

035315 

Cubic  feet 

Pounds  

.  45359 

Kilograms. 

Kilograms 

2  2046 

Pounds. 

Ounces  (avoirdupois) 

28  348 

Grams 

Grams  

.  035275 

Ounces. 

Pounds  per  square  inch 

070308 

Kilograms  per  square  cen- 

Kilograms per  square  meter  .  .  . 

14  223 

timeter. 
Pounds  per  square  inch. 

Pounds  per'  square  foot 

4  8825 

Kilograms  per  square  me- 

Kilograms per  square  centimeter 

20481 

ter. 
Pounds  per  square  foot. 

Inches  to  millimeters. 


Inch.  Millimeters. 

(0.0625) 1.  5875 

(0.1250) 3. 1750 

(0.1875) 4.  7625 

(0.2500) 6.3500 

(0.3125) 7.  9375 

(0.3750) *. 9. 5250 

(0.4375) 11.1125 

(0.5000) + 12.7000 


Inch.  Millimeters. 

^  (0.5625) 14.  2875 

f  (0.6250) 15. 8750 

•H  (0.6875) 17.4625 

|  (0.7500) 19. 0500 

-M  (0.8125) 20.  6375 

I  (0.8750) 22.2250 

H  (0.9375) 23.8125 

1  (1.0000) 25. 4000 


Millimeters  to  inches. 


Millimeter.  Inch. 

0.005 0.000196 

0.01 * 000393 

0.02 000787 

0.03 001181 

0.04...  .001574 


Millimeter. 

0.05 

0.06 

0.07 

0.08... 


Inch. 

001968 

002362 

002755 

003149 

0.09...  .003543 


MANUAL  OF   AEBOGRAPHY. 

TABLE  6. — Conversion  of  nautical  and  statute  miles. 
[1  nautical  mile  i  =6,080.27  feet.] 


157 


Nautical 
miles. 

Statute 
miles. 

Statute 
miles. 

Nautical 
miles. 

1 

1.  1516 

, 

0.8684 

2 

2.3031 

2 

1.7368 

3 

3.4547 

3 

3.  6062 

4 

4.6062 

4 

3.4736 

5 

5.  7578 

5 

4.3420 

6 

6.9093 

6 

5.2104 

7 

8.0609 

7 

6.  0788 

8 

9.2124 

8 

6.  9472 

9 

10.3640 

9 

7.8155 

i  As  denned  by  the  United  States  Coast  Survey. 

TABLE  7. — Conversion  of  velocities — Miles  per  hour  into  meters  per  second,  feet  per 
second,  and  kilometers  per  hour. 


Miles  per  hour. 

Meters 
per 
second. 

Feet  per 
second. 

Kilome- 
ters per 
hour. 

Miles  per  hour. 

Meters 
per 
second. 

Feet  per 
second. 

Kilome- 
ters per 
hour. 

0.0 

0.0 

0.0 

0.0 

22  5 

10  1 

33  0 

36  2 

05 

0  2 

0  7 

0  8 

230 

10  3 

33  7 

37  0 

1.0        

0.4 

1.5 

1.6 

23.5 

10  5 

34  5 

37  g 

1  5 

0.7 

2  2 

2  4 

24  0 

10  7 

35  2 

38  6 

2.0  

0.0 

2.9 

3.2 

24.5 

11.0 

35.9 

39  4 

2  5 

1  i 

3  7 

4  0 

25  0 

11  2 

36  7 

40  2 

3.0  

1.3 

4.4 

4.8 

25.5 

11.4 

37  4 

41  0 

3.5 

1.6 

5.1 

5  6 

26  0 

11  6 

38  1 

41  8 

40 

1  8 

5  9 

•6  4 

26  5 

11  8 

38  9 

42  6 

4.5             

2.0 

6.6 

7.2 

27.0 

12  1 

39  6 

43  5 

50 

2  2 

7  3 

8  0 

27  5 

12  3 

40  3 

44  3 

5.5 

2.5 

8.1 

8  9 

28.0 

12  5 

41  1 

45  i 

60 

2  7 

8  8 

9  7 

2S  5 

12  7 

41  8 

45  9 

6.5  

2.9 

9.5 

10.5 

29.0 

13.0 

42.5 

46  7 

7.0 

3.1 

10  3 

11  3 

29  5 

13  2 

43  3 

47  5 

7.5 

3.4 

11.0 

12.1 

30.0 

13.4 

44  0 

4«  3 

80 

3  6 

11.7 

12  9 

30  5 

13  6 

44  7 

49  1 

85 

3  8 

12  5 

13  7 

31  0 

13  9 

45  5 

49  9 

90 

4.0 

13.2 

14  5 

31.5 

14  1 

46  2 

50  7 

95 

4  2 

13  9 

15  3 

32  0 

14  3 

46  9 

51  5 

100 

4  5 

14  7 

16  1 

32  5 

14  5 

47  7 

52  3 

105 

4  7 

15  4 

16  9 

33  0 

14  8 

48  4 

53  1 

11.0         

4.9 

16.1 

17.7 

33  5 

15.0 

49  1 

53  9 

11  5 

5  1 

16  9 

18  5 

34  0 

15  2 

49  9 

54  7 

12.0  

5.4 

17.6 

19.3 

34.5 

15.4 

50.6 

55  5 

125 

5  6 

18  3 

20  1 

35  0 

15  6 

51  3 

56  3 

13.0         

5.8 

19.1 

20.9 

35.5 

15.9 

52  1 

57.1 

13.5 

6  0 

19  8 

21  7 

36  0 

16  1 

52  8 

57  9 

140 

6  3 

20  5 

22  5 

36  5 

16  3 

53  5 

58  7 

14.5       

6.5 

21  3 

23  3 

37.0 

16.5 

54  3 

59  5 

150 

6  7 

22  0 

24  1 

37  5 

16  8 

55  0 

60  4 

15.5         

6.9 

22  7 

24  9 

38  0 

17  0 

55  7 

61  2 

160 

7  2 

23  5 

25  7 

38  5 

17  2 

56  5 

62  0 

165 

7  4 

24  2 

26  6 

39  0 

17  4 

57  2 

62  8 

17.0           

7  6 

24  9 

27  4 

39  5 

17  7 

57  9 

63  6 

175 

7  8 

25  7 

28  2 

40  0 

17  9 

58  7 

64  4 

18.0 

8  0 

26  4 

29  0 

40  5 

18  1 

59  4 

65  2 

185 

8  3 

27  1 

29  8 

41  0 

18  3 

60  1 

66  0 

19  0 

8  5 

27  9 

30  6 

41  5 

18  6 

60  9 

66  8 

19.5         

8  7 

28  6 

31  4 

42  0 

18  8 

61  6 

67  6 

20.0  

8.9 

29  3 

32  2 

42.5 

19  0 

62  3 

68.4 

20.5 

9  2 

30  1 

33  0 

43  0 

19  2 

63  1 

69  2 

21  0 

9  4 

30  8 

33  g 

43  5 

19  4 

63  8 

70  0 

21.5  

9.6 

31  5 

34  6 

44  0 

19  7 

64  5 

70.8 

22.0... 

9.8 

32.3 

35.4 

44.5... 

19.9 

65.3 

71.6 

158 


MANUAL   OP   AEROGRAPHY. 


TABLE  7. — Conversion  of  velocities — Miles  per  hour  into  meters  per  second,  feet  per 
second,  and  kilometers  per  hour — Continued . 


Miles  per  hour. 

Meters 
per 
second. 

Feet  per 
second. 

Kilome- 
ters per 
hour. 

Miles  per  hour. 

Meters 
per 
second. 

Feet  per 
second. 

Kilome- 
ters per 
hour. 

45.0                     .   ... 

20.1 

66.0 

72.4 

72.5  .  .  . 

32.4 

106.3 

116.7 

45  5 

20.3 

66.7 

73.2 

73.0  

32.6 

107.1 

117  5 

46  0 

20.6 

67.5 

74.0 

73  5 

32  9 

107  8 

118  3 

46.5 

20.8 

68.2 

74.8 

74.0... 

33.1 

108.5 

119.1 

470 

21.0 

68.9 

75.6 

74.5  .... 

33.3 

109.3 

119  9 

47.5  

21.2 

69.7 

76.4 

75.0  

33.5 

110.0 

120.7 

480 

21.5 

70.4 

77.2 

75.5  

33.8 

110.7 

121  5 

485 

21.7 

71.1 

78.1 

760     .. 

34.0 

111  5 

122  3 

49  0 

21  9 

71  9 

78.9 

765 

34  2 

112  2 

123  1 

495 

22.1 

72.6 

79.7 

770  

34.4 

112.9 

123  9 

50.0  .  .  . 

22.4 

73.3 

80.5 

77.5  .  .  . 

34.6 

113.7 

124.7 

505 

22.6 

74.1 

81.3 

78.0  

34.9 

114.4 

125  5 

51  0 

22  8 

74.8 

82.1 

785 

35  1 

115  0 

126  3 

51.5  

23.0 

75.5 

82.9 

79.0  

35.3 

115.9 

127.1 

520 

23.2 

76.3 

83.7 

79.5  

35.6 

116.5 

127  9 

52.5  .  .  . 

23.5 

77.0 

84.5 

80.0... 

35.8 

117.3 

128.7 

53  0 

23  7 

77.7 

85.3 

80.5   ... 

36.0 

117.9 

129  5 

53.5  

23.9 

78.5 

86.1 

81.0  

36.2 

118.7 

130.3 

54  0 

24.1 

79.2 

86.9 

81.5.... 

36.5 

119.5 

131.1 

54  5 

24  4 

79.9 

87.7 

820 

36.7 

120  2 

131  9 

550 

24.6 

80.7 

88.5 

82.5  

36.9 

120  9 

132  7 

55  5 

24  8 

81.4 

89.3 

83  0 

37.1 

121  7 

133  5 

56.0  

25.0 

82.1 

90.1 

83.5  

37.4 

122.  4 

134.4 

565 

25.3 

82.9 

90.9 

84.0  

37.6 

123.1 

135  2 

57  0 

25  5 

83  6 

91  7 

84  5 

37  8 

123  9 

136  0 

57  5 

25.7 

84.3 

92.5 

850      . 

38.0 

124  6 

136  8 

580 

25  9 

85  1 

93  3 

855 

38  3 

125  3 

137  6 

58  5 

26  2 

85  8 

94  1 

860 

38  5 

126  1 

138  4 

590 

26.4 

86.5 

95.0 

865   . 

38  7 

126  8 

139  2 

59  5 

26  6 

87  3 

95  8 

870 

39  0 

127  5 

140  0 

600 

26  8 

88.0 

96.6 

87  5 

39  2 

128  3 

140  8 

60.5  ... 

27.0 

88.7 

97.4 

88.0  

39.4 

129.0 

141.6 

61  0 

27.3 

89.5 

98.2 

885   .. 

39.6 

129  7 

142  4 

61  5 

27  5 

90  2 

99.0 

890 

39  8 

130  5 

143  2 

62.0  .   .   . 

27.7 

90.9 

99.8 

89.5  

40.1 

131.2 

144.0 

62.5... 

27.9 

91.7 

100.6 

90.0  

40.3 

131.9 

144.8 

630  

28.2 

92.4 

101.4 

90.5  

'40.5 

132.7 

145.6 

63  5 

28  4 

93.1 

102  2 

91  0 

40  7 

133  4 

146  4 

64.0  

28.6 

93.9 

103.0 

91.5  

41.0 

134.1 

147.2 

645... 

28.8 

94.6 

103.8 

92  0 

41  3 

134  9 

148  0 

65  0 

29  1 

95  3 

104  6 

92.5  

41.5 

135.6 

148.8 

65  5 

29  3 

96  1 

105  4 

93.0  

41.7 

136.3 

149.6 

66.0  

29.5 

96.8 

106.2 

93.5  

42.0 

137.1 

150.  5 

66.5  

67  0 

29.7 
SO  0 

97.5 
98  3 

107.0 
107  8 

94.5  

42  A 

138.  5 

152.1 

950 

42  6 

139  3 

152.9 

67.5... 

30.2 

99.0 

108.6 

qc  = 

68.0  

30.4 

99.7 

109.4 

96.0  

43.1 

140.6 

154.5 

69.0  

30.8 

101.2 

111.0 

96.5  
Q7  0 

43.3 

141.4 

155.3 

69.5  .  .  . 

31.1 

101.9 

111.8 

97.5  

43.8 

142.8 

156.9 

70.0... 

31.3 

102.7 

112.7 

98.0 

44  0 

143  5 

157  7 

70.5  

31.5 

103.4 

113.5 

985 

44  2 

144  3 

158  5 

71  0 

31  7 

104  1 

114  3 

99  0 

44  4 

145  0 

159  3 

71.5  

32.0 

104.9 

115.1 

995      . 

44.7 

145  7 

KiO    1 

72.0  

32.2 

105.6 

115.9 

100 

44  9 

146  4 

160  9 

MANUAL   OF   AEROGRAPHY. 


159 


TABLE  8. — Pressure. 

(Inches  of  mercury  at  273°  A.  and  45°  latitude,  to  kilobars.  For  brevity,  the  fundamental  equations  may 
he  written:  g 45-980.624  cm/sec-.  Pensity  of  mercury  at  normal  freezing  point  of  water =13.5959.  "l 
mercury-inch=33.8660  kilobars:  1  millimeter  =  1.33320  kilobars.  1,000  kilobars =29 .5306  mercury-inches = 
750.076  millimeters.] 


Kilobars. 

0.0 

0.2 

0.4 

0.6 

0.8 

10 

338.7 

345.4 

352.2 

359.0 

365.8 

11 

372.5 

379.3 

386.0 

392.8 

399.7 

12                                                                                

406.4 

413.2 

419.9 

426.7 

433.5 

13 

440.3 

447.0 

453.8 

460.6 

467.4 

14 

474.1 

481.0 

487.7 

494.4 

501  2 

1.-,                                                                            

508.0 

514.  8 

521.5 

528.3 

:>:r>.  o 

16 

541.9 

548.6 

555.4 

562.2 

:>iix  «) 

17                                                                 

575.7 

582.5 

589.3 

596.0 

602.8 

18                                                                                    .... 

609.6 

616.4 

623.1 

630.9 

637.7 

n 

644.6 

651.2 

658.0 

664.8 

671  6 

0.00 

0.02 

0.04 

0.06 

0.08 

200 

677.3 

678.0 

678.7 

679.4 

680.0 

20  1 

680.7 

681.4 

682.0 

682.7 

683  4 

•1)  •> 

684.1 

684.8 

685.5 

686  1 

686  8 

203                                                                

687.5 

688.2 

688.8 

689.5 

690.2 

20  4 

690.9 

691.5 

692.2 

692.9 

693  6 

205                                                                                    

694.3 

694.9 

695.6 

6%.  3 

697.0 

20  6 

697.6 

698.3 

699.0 

699.7 

700  4 

20  7 

701  0 

701  7 

702  4 

703  2 

703  8 

208 

704.5 

705.2 

705.9 

706.5 

707  2 

20  9 

707  9 

708.6 

709.2 

709  9 

710  6 

21  0 

711.3 

712.0 

712.6 

713.3 

714.0 

21  1 

714.7 

715.3 

716.0 

716.7 

717  4 

21  2 

718.0 

718.7 

719.4 

720  0 

720  7 

21  3                                                                             

721.4 

722.1 

722.8 

723.5 

724  1 

21  4 

724.8 

725.5 

726.2 

726  8 

727  5 

21  5                                                                    ... 

728.2 

728.9 

729.6 

730.2 

730  9 

21  6 

731.6 

732  3 

732.9 

733  6 

734  3 

21  7                                                         

735.0 

735.7 

736.3 

737.0 

737  7 

21  8 

738.4 

739.0 

739.7 

740  4 

741  1 

21  9                                                         

741.7 

742.4 

743.1 

743.8 

744  4 

220 

745.4 

746  1 

746.8 

747  4 

748  1 

22  1                                                           ....              .     .. 

748.8 

749.5 

750.1 

750.8 

751  3 

22  2 

751.9 

752  6 

753.3 

754  0 

754  6 

223                                                

755.3 

756.0 

756.7 

757.3 

758  0 

224 

758.7 

759  4 

760.1 

760  7 

761  4 

22.5 

762.1 

762.8 

763.4 

764.1 

764  8 

226 

765.5 

766  2 

766.8 

767  5 

768  2 

22.7                                                      

768.8 

769.5 

770.2 

770.9 

771  6 

228 

772.3 

772.9 

773.6 

774  3 

775  o 

22.9.                           

775.6 

776.3 

777.0 

777.7 

777  3 

230 

779  0 

779  7 

780.4 

781  0 

781  7 

23.1. 

782.7 

783.4 

784.1 

784.8 

785  4 

232 

786  1 

786  8 

787.5 

788  2 

788  8 

23.3  

789.5 

790.2 

790.9 

791.5 

792  2 

23  4 

792  9 

793  6 

794.3 

795  0 

795  6 

23.5... 

796.3 

797.0 

797.6 

798.3 

799  0 

23.6 

799.7 

800.3 

801  0 

801  7 

802  4 

£17 

803  1 

S03  7 

804  4 

805  1 

805  8 

23.8 

806.4 

806.1 

806  8 

807  5 

808  2 

23  9 

808  8 

809  5 

810  2 

810  9 

811  5 

24.0... 

812.8 

813.5 

814.1 

814  8 

815  5 

24.1 

816  2 

816  8 

817  5 

818  2 

818  9 

24.2  

819.6 

820.2 

820  9 

821  6 

822  3 

24.3 

822  9 

823  6 

824  3 

825  0 

825  7 

24  4 

826  3 

v>7  () 

827  7 

828  4 

829  0 

24.5... 

829  7 

830  4 

831  1 

831  8 

832  4 

24.6 

833  1 

833  8 

834  5 

835  1 

835  8 

24  7 

836  5 

837  2 

837  8 

838  5 

839  2 

24.8  

839  9 

840  6 

841  3 

841  9 

842  6 

24.9... 

843.3 

"43.  '.< 

844.6 

R45.  3 

846.0 

160 


MANUAL  OF   AEROGRAPHY. 


TABLE  8. — Pressure — Continued. 


Kilobars. 

Inches. 

0.00 

0.02 

0.04 

0.06 

0.08 

25  o                                                                        

84:6.7 

847.4 

848.1 

848.8 

849.5 

25  1                                   

850.1 

850.8 

851  5 

852.  2 

852.8 

25  2                                          

853.5 

854.2 

854  9 

855.6 

856.2 

25  3                                                                                   

856.9 

857.6 

858.3 

858.  9 

859.6 

254                               

860.3 

861.0 

861.7 

862.3 

863.0 

25  5                                                     .   .           

863.7 

864.4 

865.0 

865.7 

866.4 

25  6                   

867.1 

867.7 

868.4 

869.1 

869.8 

25  7                                                  

870.5 

871.1 

871.8 

872.5 

873.2 

25  8                                                                                 ... 

873.8 

874.5 

875.2 

875.9 

876.5 

259                                          

877.2 

877.9 

878.5 

879.2 

879.9 

26  0 

880.6 

881.3 

882.0 

882.6 

883.3 

26  1                                       

884.0 

884.7 

885.3 

886.0 

886.7 

26  2                                                                                  .       - 

887.4 

888.0 

888.7 

889.4 

890.1 

26  3                                          

890.8 

891.4 

892.1 

892.8 

893.5 

26  4                                                  -                                 - 

894.2 

894.8 

895.5 

896.2 

896.9 

26.5                                                    

897.5 

898.2 

898.9 

899.6 

900.2 

26  6                                                  .                                 ... 

900.9 

901.6 

902.3 

902.9 

903.6 

26.7                              

904.3 

905.0 

905.6 

906.3 

907.0 

26  8                                                         ...                  

907.7 

908.4 

909.0 

906.7 

910  4 

26  9 

911  1 

911  7 

912  4 

913  2 

913  9 

27.0 

914.3 

915.0 

915.7 

916.3 

917.0 

27  1 

917.7 

918  4 

919  0 

919.7 

920  4 

27  2 

921  1 

921  8 

922  4 

923  1 

923  8 

27.3 

924.5 

925.1 

925.8 

926.5 

927.2 

27  4 

927.9 

928  5 

929  2 

929  9 

930  6 

27  5 

931:2 

931.9 

932.6 

933  3 

933  9 

27.6.  .             

934.6 

935.3 

936.0 

936.7 

937.3 

27.7                                                     

938.0 

938.7 

939.4 

940.0 

940.7 

27  8 

941  4 

942  1 

942  8 

943  4 

944  1 

27.9..                

944.8 

945.5 

946.1 

946.8 

947.5 

280 

948  2 

948  8 

949  5 

950  2 

950  9 

28.1..                

951.6 

952.2 

952.9 

953  6 

954  3 

28  2 

954  9 

955  6 

956  3 

957  0 

957  7 

28.3  

958.3 

959.0 

959.7 

960.4 

961.0 

28  4 

961.7 

962  4 

963  1 

963  7 

964  4 

28.5..     . 

965.1 

965.8 

966  5 

967  1 

967  8 

28.6 

968  5 

969  2 

969  8 

970  5 

971  2 

28  7 

971  9 

972  6 

973  2 

973  9 

974  6 

28.8.. 

975.3 

975  9 

976  6 

977  3 

978  0 

28.9 

978  6 

979  3 

980  0 

980  7 

981  4 

29.0..                

982  0 

982  7 

983  4 

984  1 

984  7 

29  1 

985  4 

986  1 

986  8 

987  5 

988  1 

29.2  

988.8 

989  5 

990  2 

990  8 

991  5 

29.3. 

992  2 

992  9 

993  5 

994  2 

994  9 

29  4 

995  6 

996  3 

996  9 

997  6 

998  3 

29.5 

999  0 

999  6 

1  000  3 

001  0 

1  001  7 

29.6  

1.002.4 

1  003  0 

1,003.7 

'004  4 

1  005  1 

29.7..                  

1  005  7 

1  006  4 

1  007  1 

007  8 

1  008  4 

29.8  

1.009.1 

1  009  8 

1,010.5 

'oil  2 

1,011  8 

29.9  

1  012  5 

1  013  2 

1  013  9 

014  5 

1  015  2 

30.0  . 

1  015  9 

016  6 

1  017  3 

017  9 

1  018  6 

30.1  

1  019  3 

'  020  0 

l'o20  6 

'021  3 

1  022  0 

30.2.. 

1  022  7 

023  3 

1  024  0 

'  024'  7 

1  025  4 

30.3 

1  026  1 

026  7 

1*027  4 

028  1 

1  '  028  8 

30.4  

l'029  4 

'  030  1 

1  030  8 

031  5 

1  032  2 

30.5.. 

1  032  8 

033  5 

1  034  2 

034  9 

1  035  5 

30.6 

1  036  2 

'  036  9 

1  037  6 

038  2 

1  038  9 

30.7..   . 

1  039  6 

040  3 

1  041  0 

'041  6 

1*042  3 

30.8. 

1  043  0 

043  7 

l'o44  3 

045  0 

1  '  045  7 

30.9  

1  046  4 

047  1 

1  047  7 

1  048  4 

l'o49  1 

NOTE. — The  value  for  gravity  is  that  of  the  United  States  Coast  and  Geodetic  Sur- 
vey. A  value,  980.665,  given  by  the  Bureau  of  Standards  was  adopted  in  1888  by 
the  International  Committee  on  Weights  and  Measures  and  has  since  been  continued 
for  convenience,  although  it  is  a  conventional  standard  and  not  exactly  equal  to  the 
value  of  45°.  There  has  been  a  slight  change  in  the  value  for  the  density  of  mercury. 
The  differences  are  small.  See  Monthly  Weather  Review  for  April,  1914,  p.  230, 
article  by  R.  N.  Covert. 


CHAPTER  XVII. 


BIBLIOGRAPHY. 


50821—18 11  161 


CHAPTER  XVII. 
BIBLIOGRAPHY. 

'    C-ARl'KNTKK,     F()KD    A.  I 

The  Aviator  and  the  Weather  Bureau  (1917),  San  I>i«-go  Chamber  of  Commerce, 

San  Diego,  Cal. 
CAVK.  C.  J.  P.: 

Structure  of  the  Atmosphere  il!H2..  Cambridge  t'niversity   Press.  Cambridge, 
Knirland. 
Chapter  I.  The  structure  of  the  atmosphere  as  disclosed  by  observation  of 

pilot  balloons. 
Chapter  V.  Summary  of  results  and  the  relation  of  the  wind  to  tin-  surface 

pressure  distribution. 

Chapter  VII.  The  wind  of  the  stratosphere. 
Chapter  IX.  General  results;  relation  of  vertical  wind  distribution  to  surface 

pressure  distribution. 
Monthly  Weather  Review  (1914),  page  7,  Weather  Bureau,  Washington,  D.  C. 

Winds  in  free  air. 
CLAYTON.  If.  If.: 

Annals  Astronomical  Observatory,  Harvard  College,  volume  :i(),  Part  IV  iisur,,, 
Cambridge,  Mass. 

Discussion  of  the  cloud  observations  made  at  Blue  Hill  Observatory. 
DINES,  W.  II.: 

Xature,  London,  volume  99  (1917),  page  125  and  pages  424-425.     Meteorology  and 

aviation. 
Xature,  London,  volume  99,  March  8,  1917,  page  24.     Horizontal  temperature 

gradient  and  the  increase  of  wind  with  height. 
Monthly  Weather  Review  (1916),  Weather  Bureau,  Washington,  D.  C.,  page  182 

(March).     The  local  circulation  of  the  atmosphere. 
DIXIE,  A.  E.,  Lieutenant  Commander,  R.  X.: 

Air  Navigation  for  Flight  Officers  (1917),  Grieves  Publishing  Co.  (Ltd.),  Ports- 
mouth, England. 
Chapter  VI.  Meteorology. 

Chapter  VII.  General  weather  in  the  British  Isles. 
Chapter  VIII.  Forecasting  by  solitary  observer. 
Appendix.  Tables  and  formulae. 
GREGG,  WILLIS  RAY: 

Monthly  WTeather  Review,  January,  1918. 

Mean  values  of  free  air  barometric  and  vapor  pressures,  temperatures,  and 

densities  over  the  United  States. 
The  turning  of  winds  with  altitude. 
HAMMELL,  GUSTAV: 

Flying;  some  practical  experiences  (1916),  Longmans,  Green  &  Co.,  London  and 
New  York. 

Chapter  VIII.  Winds,  eddies,  and  other  disturbances. 
Chapter  IX.  Weather. 
II ANN-,  JULIUS: 

Lehrbuch  der  Meteorologie,  third  edition.  Charles  Hermann  Tauehnitx.  Leipzig. 
Introduction.  Atmosphere  in  general. 
( 'hapter  I.  Temperature. 
Book  II,  Chapter  I.  Pressure. 
Book  V.  Chapter  V.  Atmospheric  elertri<  ity. 

1  <;:; 


164  MANUAL   OF  AEKOGKAPHY. 

HUMPHREYS,  W.  J.: 

The  Thunderstorm  and  its  Phenomena  (1914),  Lippincott,  New  York  City. 
Monthly  Weather  Review  (1914),  page  348,  Washington,  D.  C.     The  thunder- 
storm and  its  phenomena. 

Monthly  Weather  Review  (191(5).     Wind  and  altitude. 
Journal  of  the   Franklin  Institute,  August,  1917,  to  January,  1918.     Physics  of 

the  air. 
MARVIN,  C.  V.: 

Monthly  Weather  Review  (1909),  volume  37,  pages  3-9.     Pressure  of  saturated 

vapor  from  water  and  ice  as  measured  by  different  authorities. 
Circular  F.  Instrument  Division,  United  States  Weather  Bureau.     Barometers 

and  the  measurement  of  atmospheric  pressure. 
McADiE,  ALEXANDER  G: 

Principles  of  A  orography  (1917);  Rand,  McNally  &  Co.,  New  York  City  and 
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Chapter  II.  Units  and  symbols. 
Chapter  III.  Temperature  scales. 
Chapter  VI.  The  circulation  of  the  atmosphere. 
Chapter  VII.  The  major  circulations. 
Chapter  VIII.  The  minor  circulations. 
Chapter  IX.  Forecasting  storms. 
Chapter  X.  The  winds. 

Chapter  XI.  The  water  vapor  of  the  atmosphere. 
Appendix.  Tables. 

Winds  of  Boston  (1916);  Harvard  University  Press,  Cambridge,  Mass.     A  classifi- 
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Annals  of  the  Harvard  College  Observatory,  volume  73,  Part  III  (1910),  Harvard 

University  Press,  Cambridge,  Mass.     Temperature  scales. 

Scientific  American  Supplement,  June  2,  1917,  Munn  &  Co.,  New  York.     Avia- 
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MILHAM,  W.  I.: 

Meteorology  (1912),  MacMillan  Co.,  New  York. 

Chapter  III.  Observation  and  distribution  of  temperature. 
Chapter  IV.  Pressure  and  circulation. 
Chapter  V.  The  moisture  in  the  atmosphere. 
Chapter  VI.  Secondary  circulation  of  the  atmosphere. 
'  Chapter  VIII.  Weather  predictions. 
Chapter  XI.  Atmospheric  electricity. 
Chapter  XII.  Atmospheric  optics. 

MOEDEBECK,  HERMANN,  W.  L.  (1907) ;  Whittaker  &  Co.,  London. 
Pocketbook  of  aeronautics. 

Chapter  II.  Physics  of  the  atmosphere. 

Chapter  III.  Meteorological  observations  in  balloon  ascents  and  the  compu- 
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MOORE,  WILLIS  L.: 

Descriptive  meteorology  (1910).     American  Book  Company,  New  York. 
Chapter  X.  The  winds  of  the  globe. 
Chapter  XIV.  Optical  phenomena  in  meteorology. 

R<>T .  H.  A.  LAWRENCE  (1909),  Moffat,  Yard  &  Co.,  New  York.     Conquest  of  the  Air. 
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MANUAL   OF   AEROGRAPH  V.  165 

SH.\\V.  \V.  X.: 


Weath-r    I'Jll  >,  l>.  Van  N  A   York. 

II.    Mela!  ion  «.f  wind  to  banum-lni-  prepare. 

<'hapter  I\".   Type-  •  aii<l  tin-  future  outlook  in  \\ealher 

Chapter  XVI.    1'pp-- 

The  \\VatherMap    1916  .  Meteoro!,.^.  -al  otfi.-e.  London. 
Barometer  Manual. 

ission  of  atmospheric   pressure. 
>\,  ( 
Quarterly  Journal  Il,,\al  Met  .don. 

Atmosphen     Kler'riritv. 
S-n.\\  \i;  i  ,  (  '.  I).  : 

Quarterly  Journal  (!'.»  17     K..\al  M«-tcn!  ..    I.on<l<ni.     Atmospheric 

electricitiral  plu'in-mcna  (luring  i. 
TAYLOB,  G.  I..  Maj.  II.  I 

Journal  of  Royal  M  .  July.  !<H7:  London.     The  formation  of 

WALHO.  FRANK: 

Kl-nu-ntary  M.-T.v.rol..::;.  a  Book  <         <  in<  innati  and  Xcu   York. 

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Chapter  XI.  \\Vathrr  and  \v«-athiT  pr«-«li<  -lions. 
WARP.  ROBERT  I>i   • 

Monthly  Weather  Kc\i--u.  Iw,-m!M-r.   l'.»17.     M<-tcoroloLr>  and  \varil;. 

Monthly  Weather  II  A  «  la^sifiration  of  methods  of  tran- 

sition from  rain  to  blur 
WAI.KI>K\.  S.  L.  : 
Aeroplane* 

<'hapter  I.  Measure  of  a  urust. 
Chapter  II.  Aeroplane.-*  in  mists. 
Chapter  IV.  Si  ni'-run-  of  mists. 

PUBLICATIONS. 

Compu  London. 

C.  <«.  >    I'nits  of  in-  ;'h  their  abbreviatioiiB  and  «'()uivalent~. 

Tat 
Observers'  Handbook,  M.  <  '  .ical  (  MHce.  London. 

«  on  use  of  instnunents  at  normal  rlimatological  station. 
Self-recording  instrumei 
Auroras  and  St.  Elmo's  fire. 
Optical  phenomena. 
Monthly  Weather  Review: 

October,  1917.  <  'lim.  i,<  »•  aiul  Uel^ium.     Articles  on  wind-        >•  •••  Tabl«- 

of  Conteir 

March.  l'.)Hi.     Paj?e  «i7'J.  I  >iurnal  pressure  changes. 
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January.  KH7.  rluiids  and  cloud  form- 
Report   i:i.  National   A«l.  917), 

National  Advisory  Coniu:  -      iHyl'.l. 

Smithsonian  Miscellane<.us  (  olle.-tion.   volun  I'.thi.     Smithsonian 

Insiitute.  \Vashin-ton.  ! 

Description  of  i  1  dis<-ussion  of  radiation  measurements. 

Weather  Forecastinu'  in  the  Cnite<l  --ather  P.ureau.  \\ashiiiK- 

ton    I'    (        ^  '      x'tnnient    Printin<:  Office:  also  Suppleniei 

Monthly  Weather  i 

.(.    P..   A. 


